A prisoner put in with a strangler (8)
I believe the answer is:
anaconda
'strangler' is the definition.
(I've seen this before)
'a prisoner put in with a' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'put in' means one lot of letters goes inside another.
'with' becomes 'and'.
'a'+'con'='acon'
'acon' placed within 'and' is 'anacond'.
'anacond'+'a'='ANACONDA'
(Other definitions for anaconda that I've seen before include "Large South America snake" , "crawler" , "Aquatic snake" , "South American killer" , "cold-blooded killer" .)