I have an exterior drainpipe that's leaking at a joint?

I have an exterior plastic drainpipe that's at a 45 degree angle, and just takes waste water from my bathroom sink, at the bottom of the slope it joins a bigger drainpipe that is vertical. At the point where two different parts of the angled pipe meet, and there is a plastic collar joining the two parts of pipe, it's leaking. I can't see how I can take out the part or remove the collar without breaking the collar/join, or without removing the whole drainpipe system right up to the sink itself. Any tips? I'm pretty much a novice at this sort of stuff, so if you could type slowly, it would be much appreciated.