Fine-tune commercial exactly (6)
I believe the answer is:
adjust
'fine-tune' is the definition.
(I've seen this before)
'commercial exactly' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'exactly' becomes 'just' (synonyms).
'ad'+'just'='ADJUST'
(Other definitions for adjust that I've seen before include "Alter by small amount to make right for use" , "carry out minor alterations" , "Adapt to new conditions" , "Adapt (oneself)" , "Regulate, alter by small amount" .)