Escape using muscles: prisoner departs (7)
I believe the answer is:
abscond
'escape' is the definition.
(I've seen this before)
'muscles prisoner departs' is the wordplay.
'muscles' becomes 'abs' (short for abdominal muscles).
'prisoner' becomes 'con' (abbreviation for 'convict').
'departs' becomes 'd' (abbreviation used in transport schedules).
'abs'+'con'+'d'='ABSCOND'
'using' acts as a link.
(Other definitions for abscond that I've seen before include "make quick getaway" , "Steal away" , "Flee" , "Run away secretly" , "Make off, bolt" .)