Ok, so now that my mast is back up and furler fixed....I have noticed that a winch is a bit loose at its base. Is this as simple as taking the retaining "circlip off, pulling it out and tightening the screws?
Sort of. Depends on how they are mounted. Ours are bolted, which means you have to get access the the back side to hold the nuts to tighten them. I would bet that most are bolted, not screwed into fiberglass. As far as the bolt heads go, our Lewmars are only accessible by lifting the housing off, as you suggest. You'll find diagrams for many Lewmars on the net if you need one.
I second arf145's advise, but before you go to the trouble of disassembling the winch...
If you can get at the bolt and nut underside (I agree they will not likely be screwed to the coaming) apply a socket and ratchet wrench to the nut. You may not need to access the head of the bolt to hold it against the torque of your wrench. In my own case, my Lewmars lock their mounting bolts from within their base, making the second wrench unnecessary.
Ah, a mast winch. Sorry, I should have got that from your question. I'm unfamiliar with how those are attached. I would guess it could be screwed into the aluminum. I guess you'll know if you can tighten the loose screw/bolt.
Yes, you can take the circlip off and pull the drum (and perhaps some internal gears) to get access to the screw heads. Plan on regreasing before reassembly.
If you are the unlucky sort, you will find that the threads in the aluminum have corroded and failed and that is why the winch is loose. Be prepared to either relocate the winch or go one screw size up and retap the mast. When you finally reassemble, be sure to use tefgel or lanacote liberally on the screws and under the winch body to minimize future corrosion.