Scrounge penny once in prison (5)
I believe the answer is:
cadge
'scrounge' is the definition.
(synonyms)
'penny once in prison' is the wordplay.
'penny once' becomes 'd' (abbreviation for pre-decimal penny).
'in' is an insertion indicator.
'prison' becomes 'cage' (cage is a kind of prison).
'd' placed into 'cage' is 'CADGE'.
(Other definitions for cadge that I've seen before include "Sponge (eg money)" , "Scrouge, ask for" , "Caged (anag.)" , "Bum" , "get free" .)