Like a fiddle? (3)
I believe the answer is:
fit
'like a fiddle?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fit that I've seen before include "Healthy, exercised" , "Suitable; in good health" , "In good physical condition, healthy" , "Be the right size or shape" , "Suitable; healthy" .)