Well-muscled enthusiast (4)
I believe the answer is:
buff
'enthusiast' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for buff that I've seen before include "Polish; yellowish-beige" , "fan of > nudism?" , "Blow" , "Yellowish colour - polish - expert" , "Yellowish-beige colour" .)