Before we get into this, understand: you are not a combatant. If Knights are front-liners, and Clerics and their ilk are second-string, you're assuredly third rank. Stay out of combat if at all possible.
Now we'll discuss your options.
In melee, your primary skill starts out at 14. This gives you a parry of 10, possibly with a +1 from a buckler. Frankly, if you're forced into combat and are the focus of something's aggression, your best bet is to take All-Out Defense for the +2 to your defenses, retreat for another +1 to parry and +3 to dodge, and get out of there.
If you aren't the focus of something's aggression, what are you doing in combat? Oh well, we'll assume you find yourself in that situation anyway. Your primary task will probably be to soak up a defense roll so the Knight or Barbarian can land a hit. If you're certain you won't be the focus of retaliation, you should go ahead and take All-Out Attack (Determined) and trade the +4 for a -2 to your foe's defense. Or, for heavily armored foes, don't forget All-Out Attack (Strong) for the extra damage.
For ranged combat, your primary skill is a 13 or 12. You must aim in order to hit anything. This means you'll be shooting every two to four seconds (most favorable is with Fast-Draw (Knife), least is with a Bow and without Fast-Draw (Arrow)). That sounds grim, but let's be clear: this is where you should be. You should never be in melee if you can help it, and if you're not slinging songs about (which, frankly, is a perfectly good thing to be doing - it's your schtick) you might as well be shooting things.
Your primary goal in ranged combat is to soak up defenses so someone actually good at fighting can hit something. Don't bother aiming for a specific location unless you need to to be considered a threat. Just aim, subtract the range penalty, and fire.
You're a second-tier fighter, nowhere near as good as a Knight or Barbarian but still not totally helpless in a fight. With a skill of 14 in both weapon and shield, you can hit your opponent %90 of the time and defend competently. Congratulations; you meet the baseline required to be considered a threat. Unless you have compelling reason to do otherwise, aim for the torso. Don't worry if they defend against you - unless they're dodging, that's one more defense they can't use against the Knight.
On the defensive, always remember to retreat for the extra +1/+3. With a shield, your defense is 12 or 13, which gives you a %90 chance of success with the retreat. If facing opponents doing Deceptive Attacks, switch to All-Out Defense - as a cleric, you want to keep your FP instead of using it for Extra Effort. Someone will be along to help you shortly, since a cleric under attack is a high priority issue.
Let's talk Sunbolt. This is your go-to ranged option. The most important thing about Sunbolt is, you want it to hit. Unlike the Scout, for whom landing arrows is nice, for you landing Sunbolt is necessary. You spent a lot of energy and time to get it up, and all that's wasted if you miss or your enemy defends against the attack.
This means two things: always Aim for at least one second to get the +2 Acc, and pick a target that's already being harrassed or otherwise can't defend well. Don't bother with aiming for the face or eyes; the special effects are a trap.