If you bulge here, that's fine (5)
I believe the answer is:
swell
This clue might be a double definition.
'if you bulge here' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'fine' is the second definition.
(I've seen this before)
'that's' is the link.
Can you help me to learn more?
(Other definitions for swell that I've seen before include "Expand abnormally" , "Gentle sea movement" , "Inflate - fine" , "wonderful" , "blowing top" .)