Keen fellows put in muscle (6)
I believe the answer is:
lament
'keen' is the definition.
('lament' can be a synonym of 'keen')
'fellows put in muscle' is the wordplay.
'fellows' becomes 'men' (I've seen this before).
'put in' is an insertion indicator.
'muscle' becomes 'lat' (lat is a kind of muscle).
'men' put into 'lat' is 'LAMENT'.
(Other definitions for lament that I've seen before include "Express sorrow" , "complaint" , "requiem" , "Mourn in mental way (6)" , "Mental (anag)" .)