One who fancies commercial muck, right? (7)
I believe the answer is:
admirer
'one who fancies' is the definition.
(someone who admires or fancies someone else)
'commercial muck right?' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'muck' becomes 'mire' (mire can mean muck or dirt).
'right?' becomes 'r' (common abbreviation).
'ad'+'mire'+'r'='ADMIRER'
(Other definitions for admirer that I've seen before include "Supporter or fan" , "One who cherishes, like boy friend" , "Married to suitor" , "A young lady's beau - he thinks she's great" , "Married (anag) -- lover" .)