Scuttled bridges to sell down the river (3,2)
I believe the answer is:
rat on
'sell down the river' is the definition.
I can't judge whether this defines the answer.
'scuttled bridges to' is the wordplay.
'scuttled' becomes 'ran'.
'bridges' is an insertion indicator (some letters go around or bridge others).
'ran' placed around 'to' is 'RAT ON'.
Can you help me to learn more?
(Other definitions for rat on that I've seen before include "grass up" , "desert" , "Give away" , "shop" , "Betray" .)