Dictator’s supporter working to infiltrate crooked parties (9)
I believe the answer is:
peronista
'dictator's supporter' is the definition.
(supporter of Juan Peron)
'working to infiltrate crooked parties' is the wordplay.
'working' becomes 'on'.
'to infiltrate' indicates putting letters inside.
'crooked' is an anagram indicator.
'parties' with letters rearranged gives 'perista'.
'on' going into 'perista' is 'PERONISTA'.
(Other definitions for peronista that I've seen before include "Follower of Juan" , "supporter of former South American president" .)