Fraud squad's head about to get married (4)
I believe the answer is:
scam
'fraud' is the definition.
(scam is a kind of fraud)
'squad's head about to get married' is the wordplay.
'head' says to take the initial letters.
'about to' becomes 'ca' (I've seen this in other clues).
'get' means one lot of letters go next to another.
'married' becomes 'm'.
The first letter of 'squads' is 's'.
's'+'ca'+'m'='SCAM'
(Other definitions for scam that I've seen before include "Racket" , "Con trick" , "Fraud, trick (slang)" , "Fraudulent scheme, swindle" , "rook" .)