A criminal entering with a snake (8)
I believe the answer is:
anaconda
'snake' is the definition.
(anaconda is a kind of snake)
'a criminal entering with a' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'entering' means one lot of letters goes inside another.
'with' becomes 'and'.
'a'+'con'='acon'
'acon' put into 'and' is 'anacond'.
'anacond'+'a'='ANACONDA'
(Other definitions for anaconda that I've seen before include "South American killer" , "cold-blooded killer" , "Snake of the boa family" , "Large South America snake" , "One can get crush on" .)