Lacking muscles, went topless (6)
I believe the answer is:
absent
'lacking' is the definition.
(I've seen this before)
'muscles went topless' is the wordplay.
'muscles' becomes 'abs' (I've seen this before).
'topless' means to remove the first letter.
'went' with its first letter removed is 'ent'.
'abs'+'ent'='ABSENT'
(Other definitions for absent that I've seen before include "Away" , "Missing or dreamy" , "elsewhere" , "nowhere to be seen" , "lack of attention" .)