Son topping academic stream is a fraud (4)
I believe the answer is:
scam
'fraud' is the definition.
(scam is a kind of fraud)
'son topping academic stream' is the wordplay.
'son' becomes 's'.
'topping' is a charade indicator (letters next to each other).
'academic stream' becomes 'cam' (River cam runs through Cambridge).
's'+'cam'='SCAM'
'is a' acts as a link.
(Other definitions for scam that I've seen before include "rook" , "Rip off" , "Hoax" , "Con trick" , "Fraudulent scheme, swindle" .)