Worker in the past isn't bad as an opponent (10)
I believe the answer is:
antagonist
'an opponent' is the definition.
('antagonist' can be a synonym of 'opponent')
'worker in the past isn't bad' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'in the past' becomes 'ago' (synonyms).
'bad' indicates anagramming the letters.
'isnt' anagrammed gives 'nist'.
'ant'+'ago'+'nist'='ANTAGONIST'
'as' acts as a link.
(Other definitions for antagonist that I've seen before include "5 down (anag)" , "Adversary" , "rival" , "One who is hostile" , "Hostile person" .)