A prisoner going in with a snake (8)
I believe the answer is:
anaconda
'snake' is the definition.
(anaconda is a kind of snake)
'a prisoner going in with a' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'going in' indicates putting letters inside.
'with' becomes 'and' (I've seen this before**).
'a'+'con'='acon'
'acon' inserted within 'and' is 'anacond'.
'anacond'+'a'='ANACONDA'
(Other definitions for anaconda that I've seen before include "Strangler" , "The largest snake" , "Large boa of South America" , "Boa constrictor" , "Large water snake" .)