Pawnbroker has a name like muck (7)
I believe the answer is:
unclean
'like muck' is the definition.
(I know that goo is unclean)
'pawnbroker has a name' is the wordplay.
'pawnbroker' becomes 'uncle' (old-fashioned term for a pawnbroker).
'has' is a charade indicator (letters next to each other).
'name' becomes 'n' (common abbreviation e.g. on forms).
'uncle'+'a'+'n'='UNCLEAN'
(Other definitions for unclean that I've seen before include "Defiled" , "Foul" , "Ceremonially impure" , "Polluted" , "Contaminated" .)