River in hard to cut channel (6)
I believe the answer is:
trough
'to cut channel' is the definition.
'river in hard' is the wordplay.
'river' becomes 'r'.
'in' is an insertion indicator.
'hard' becomes 'tough' (I've seen this before).
'r' placed within 'tough' is 'TROUGH'.
(Other definitions for trough that I've seen before include "Pigswill container" , "where animals may be drinking" , "Long narrow open receptacle for feeding cattle" , "Farm food container" , "Channel; low point" .)