Reasons chartered accountant employs (6)
I believe the answer is:
causes
'reasons' is the definition.
(the reasons for something)
'chartered accountant employs' is the wordplay.
'chartered accountant' becomes 'CA' (abbreviation for chartered accountant).
'employs' becomes 'uses' ('use' can be a synonym of 'employ').
'ca'+'uses'='CAUSES'
(Other definitions for causes that I've seen before include "'Reasons, grounds (6)'" , "Origins or crusaders" , "See 5" , "Reasons why things happen" , "They bring about effects" .)