Associate giving prisoner unfinished spliff (7)
I believe the answer is:
conjoin
'associate' is the definition.
(thesaurus)
'giving prisoner unfinished spliff' is the wordplay.
'giving' says to put letters next to each other.
'prisoner' becomes 'con' (abbreviation for 'convict').
'unfinished' means to remove the last letter.
'spliff' becomes 'joint'.
'joint' with its final letter taken away is 'join'.
'con'+'join' is 'CONJOIN'.
(Other definitions for conjoin that I've seen before include "put together" , "Team up with" .)