Gather prisoner got even, somehow (7)
I believe the answer is:
convene
'gather' is the definition.
(convening is a kind of gathering)
'prisoner got even somehow' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'got' means one lot of letters go next to another.
'somehow' indicates an anagram.
'even' with letters rearranged gives 'vene'.
'con'+'vene'='CONVENE'
(Other definitions for convene that I've seen before include "Bring together to meet" , "Bring together (for a meeting)" , "Mass" , "Meet formally" , "Call (meeting)" .)