Female prisoner heading west (3)
I believe the answer is:
gal
'female' is the definition.
(gal is a kind of female)
'prisoner heading west' is the wordplay.
'prisoner' becomes 'lag' (slang term).
'heading west' shows that the letters should be reversed in order.
'lag' in reverse letter order is 'GAL'.
(Other definitions for gal that I've seen before include "Young woman (colloq.)" , "female noted in Kalamazoo" , "An American girl, cowboy vintage" , "Lass" , "US female" .)