One can't fly? Try cycling! (4)
I believe the answer is:
rhea
'fly? try cycling' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe there's an association between them I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for rhea that I've seen before include "Large S. American bird" , "Flightless bird - Greek fertility goddess" , "Ostrich-like bird of South America" , "Fertility goddess in ancient Greek mythology" , "nonflyer" .)