Not on duty the worker is casual (7)
I believe the answer is:
offhand
'casual' is the definition.
('offhand' can be a synonym of 'casual')
'not on duty the worker' is the wordplay.
'not on duty' becomes 'off'.
'the worker' becomes 'hand' (hand is a kind of worker**).
'off'+'hand'='OFFHAND'
'is' acts as a link.
(Other definitions for offhand that I've seen before include "Of speech, casual or curt" , "Rudely casual; impromptu" , "Without consideration" , "Casual; unconcerned" , "Dismissive; curt" .)