Birds share nuts (5)
I believe the answer is:
'birds' is the definition.
(rhea is a kind of bird)
'share nuts' is the wordplay.
'nuts' is an anagram indicator (nuts can mean crazy).
'share' is an anagram of 'RHEAS'.
(Other definitions for rheas that I've seen before include "these birds won't fly" , "Ostrich-like birds" , "Flightless birds" .)