Maintain a fine business (6)
I believe the answer is:
affirm
'maintain' is the definition.
(I've seen this before)
'a fine business' is the wordplay.
'fine' becomes 'f' (abbreviation).
'business' becomes 'firm' (firm is a kind of business).
'a'+'f'+'firm'='AFFIRM'
(Other definitions for affirm that I've seen before include "Declare, assert" , "Emphatically state" , "State as a fact, perhaps in court" , "Ratify; vouch" , "Avow or corroborate" .)