Required fellow attorney to join politician (9)
I believe the answer is:
mandatory
'required' is the definition.
('mandatory' can be a synonym of 'required')
'fellow attorney to join politician' is the wordplay.
'fellow' becomes 'man' (I've seen this before**).
'attorney' becomes 'da' (District Attorney).
'to join' is a charade indicator (letters next to each other).
'politician' becomes 'tory' (I've seen this before).
'man'+'da'+'tory'='MANDATORY'
(Other definitions for mandatory that I've seen before include "Not optional" , "Binding" , "allowing no option" , "required" , "Obligatory" .)