Once river has risen, catches flightless birds (5)
I believe the answer is:
rheas
'flightless birds' is the definition.
(rhea is a kind of flightless bird)
'once river has risen catches' is the wordplay.
I cannot really understand how this works, but
'river' could be 'ea' (I've seen this in other clues) and 'ea' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rheas that I've seen before include "these birds won't fly" , "Flightless birds" , "Ostrich-like birds" .)