Unexpectedly greet a bird (5)
I believe the answer is:
egret
'a bird' is the definition.
(I know that egret is a type of heron)
'unexpectedly greet' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'greet' is an anagram of 'EGRET'.
(Other definitions for egret that I've seen before include "White-feathered wading bird" , "Seabird" , "white fish eater" , "flier" , "young bird" .)