Film villain's casual task (6)
I believe the answer is:
oddjob
'film villain's' is the definition.
(James Bond villain)
'casual task' is the wordplay.
'casual' becomes 'odd' (I've seen this before).
'task' becomes 'job' (synonyms).
'odd'+'job'='ODDJOB'
(Other definitions for oddjob that I've seen before include "casual work" , "Villainous character" , "henchman in film" .)