Rotter has denied beginning long con (7)
I believe the answer is:
swindle
'con' is the definition.
(both can mean a fraud or scam)
'rotter has denied beginning long' is the wordplay.
'rotter' becomes 'swine' (both can mean a horrible person).
'has' is an insertion indicator.
'beginning' indicates taking the first letters.
'long' becomes 'l'.
The initial letter of 'denied' is 'd'.
'd'+'l'='dl'
'swine' enclosing 'dl' is 'SWINDLE'.
(Other definitions for swindle that I've seen before include "Fraudulent action" , "Fraudulent scheme" , "Cheat (out of money)" , "Scam" , "Diddle, sting, twist" .)