Purpose applied to accountant's campaign (5)
I believe the answer is:
cause
'campaign' is the definition.
('cause' can be a synonym of 'campaign')
'purpose applied to accountant's' is the wordplay.
'purpose' becomes 'use' ('use' can be a synonym of 'purpose').
'applied to' means one lot of letters go next to another.
'accountant' becomes 'ca' (abbreviation for chartered accountant).
'use' put after 'ca' is 'CAUSE'.
(Other definitions for cause that I've seen before include "Grounds" , "Bring about" , "Make happen; something deserving support" , "why some people fight" , "Reason; belief" .)