Not on duty a worker becomes casual (7)
I believe the answer is:
offhand
'casual' is the definition.
(synonyms)
'not on duty a worker' is the wordplay.
'not on duty' becomes 'off'.
'a worker' becomes 'hand' (hand is a kind of worker).
'off'+'hand'='OFFHAND'
'becomes' acts as a link.
(Other definitions for offhand that I've seen before include "Ungraciously curt" , "Cool in manner" , "Casual; unconcerned" , "Casually rude" , "Ungraciously cool" .)