They're horrible river swimmers (7)
I believe the answer is:
rotters
'swimmers' is the definition.
The answer and definition can be both people as well as being plural nouns.
Maybe you can see an association between them that I can't see?
'they're horrible river' is the wordplay.
I cannot quite understand how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found in the answer.
The remaining letters 'otters' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for rotters that I've seen is " Contemptible folk".)