Crossword Genius

Try to move river to the west, but it won't fly (4)

Ross

I believe the answer is:

rhea

I'm a little stuck... Click here to teach me more about this clue!

'fly' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'try to move river to the west but it won't' 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 found within the answer.
The remaining letters 'rh' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for rhea that I've seen before include "Titaness" , "Ostrich-like bird of South America" , "Grounded bird" , "nonflyer" , "Flightless bird - Greek fertility goddess" .)

I've seen this clue in The Independent.
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