Bodybuilder's bane (4)
I believe the answer is:
flab
'bodybuilder's bane' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flab that I've seen before include "Soft loose flesh" , "Ugly, surplus flesh" , "Sign of overeating" , "Excess body fat" , "Loose flesh; fat" .)