Lack of muscles means crew regularly lost (7)
I believe the answer is:
absence
'lack' is the definition.
(I know that lack can be written as absence)
'muscles means crew regularly lost' is the wordplay.
'muscles' becomes 'abs' (short for abdominals).
'regularly lost' indicates alternate letters (letters removed at regular intervals).
'means'+'crew'='meanscrew'
The alternate letters of 'meanscrew' are 'ence'.
'abs'+'ence'='ABSENCE'
'of' is the link.
(Other definitions for absence that I've seen before include "Failure to attend" , "form of inattention?" , "It makes the heart grow fonder, they say" , "Want, lack" , "Truancy" .)