Emphatically endorse a fine business (6)
I believe the answer is:
affirm
'emphatically endorse' is the definition.
'affirm' can be an answer for 'endorse' (I've seen this before). I am not sure about the 'emphatically' bit.
'a fine business' is the wordplay.
'fine' becomes 'f' (abbreviation).
'business' becomes 'firm' (firm is a kind of business).
'a'+'f'+'firm'='AFFIRM'
Can you help me to learn more?
(Other definitions for affirm that I've seen before include "Confidently put" , "Uphold" , "Make a formal declaration (rather than an oath)" , "Declare, assert" , "Declare one's commitment (to)" .)