River mammals in river, snakes in the grass (7)
I believe the answer is:
rotters
'river snakes in the grass' is the definition.
I can't tell whether this defines the answer.
'river mammals' is the wordplay.
I cannot really see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is located in the answer.
'mammals' could be 'otters' (otter is a kind of mammal) and 'otters' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Another definition for rotters that I've seen is " Contemptible folk".)