Change commercial completely (6)
I believe the answer is:
adjust
'change' is the definition.
(to adjust is to change)
'commercial completely' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'completely' becomes 'just' (I can't explain this - if you can you should believe this answer much more).
'ad'+'just'='ADJUST'
Can you help me to learn more?
(Other definitions for adjust that I've seen before include "Adapt (oneself)" , "Regulate, alter by small amount" , "Adapt, fix" , "Arrange, put into proper position" , "Adapt (to new situation)" .)