Attacker regularly causes trouble with worker (9)
I believe the answer is:
assailant
'attacker' is the definition.
('assailant' can be a synonym of 'attacker')
'regularly causes trouble with worker' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
'with' says to put letters next to each other.
'worker' becomes 'ant' (worker is a type of ant).
The alternate letters of 'causes' are 'ass'.
'ass'+'ail'+'ant'='ASSAILANT'
(Other definitions for assailant that I've seen before include "Attacking person" , "Aggressive sort" , "Who would tear into" , "One attacking another" , "Aggressor" .)