Crossword Genius

Well-muscled enthusiast (4)

Ross

I believe the answer is:

buff

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Times and The Sun.
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