Broad lout backing spiv (4,3)
I believe the answer is:
wide-boy
'spiv' is the definition.
(I've seen this before)
'broad lout backing' is the wordplay.
'broad' becomes 'wide' (synonyms).
'lout' becomes 'yob' (both can mean a rude or criminal young person).
'backing' says the letters should be written in reverse.
'yob' in reverse letter order is 'boy'.
'wide'+'boy'='WIDE-BOY'
(Other definitions for wide-boy that I've seen before include "Petty criminal" , "Spiv" .)