A Green unexpectedly showing anger (6)
I believe the answer is:
enrage
'anger' is the definition.
(to enrage is to anger)
'a green unexpectedly' is the wordplay.
'unexpectedly' indicates an anagram.
'a'+'green'='agreen'
'agreen' is an anagram of 'ENRAGE'.
'showing' is the link.
(Other definitions for enrage that I've seen before include "Drive to anger" , "Make violently angry" , "Genera (anag.)" , "put in a stir?" , "get annoyed" .)