Supposes old muscles take time (7)
I believe the answer is:
expects
'supposes' is the definition.
(I've seen this before)
'old muscles take time' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'muscles' becomes 'pecs' (short for pectoral muscles).
'take' indicates putting letters inside.
'time' becomes 't' (abbreviation).
'pecs' placed around 't' is 'pects'.
'ex'+'pects'='EXPECTS'
(Other definitions for expects that I've seen before include "'England - - ...' (Nelson)" , "Looks forward to" , "Anticipates" , "Supposes" , "Assumes" .)