Back with fish to sell down the river (6)
I believe the answer is:
betray
'sell down the river' is the definition.
(I've seen this before)
'back with fish' is the wordplay.
'back' becomes 'bet'.
'with' says to put letters next to each other.
'fish' becomes 'ray' (ray is a kind of fish**).
'bet'+'ray'='BETRAY'
'to' acts as a link.
(Other definitions for betray that I've seen before include "26 [SELL]" , "Act treacherously towards (someone)" , "Double-cross like Judas" , "Breach the trust of" , "Give away" .)