Large birds running around like hares (5)

I believe the answer is:
rheas
'large birds' is the definition.
(I've seen this in another clue)
'running around like hares' is the wordplay.
'hares' anagrams to 'RHEAS'.
Though, I'm unsure how this is indicated.
Can you help me to learn more?
(Other definitions for rheas that I've seen before include "Ostrich-like birds" , "these birds won't fly" , "Flightless birds" .)
