Some muscles not present? Not half (3)
I believe the answer is:
abs
'some muscles' is the definition.
(short for abdominals)
'not present? not half' is the wordplay.
'not present?' becomes 'absent' ('absent' is the opposite to 'present').
'not half' indicates taking half.
'absent' cut in half is 'ABS'.
(Other definitions for abs that I've seen before include "Exercise to improve these" , "Stomach muscles (abbr)" , "Some musculature" , "stopping system" , "ratings?" .)