Accountant has added a charge on a whim (7)
I believe the answer is:
caprice
'whim' is the definition.
(I know that whim can be written as caprice)
'accountant has added a charge' is the wordplay.
'accountant' becomes 'CA' (abbreviation for chartered accountant).
'has added' is a charade indicator (letters next to each other).
'a charge' becomes 'price' (I've seen this in another clue).
'ca'+'price'='CAPRICE'
'on a' acts as a link.
(Other definitions for caprice that I've seen before include "unaccountable change" , "Sudden mood change" , "Sudden whim" , "Whimsy" , "Sudden change of mind" .)