Infuriate a Green dreadfully (6)
I believe the answer is:
enrage
'infuriate' is the definition.
(thesaurus)
'a green dreadfully' is the wordplay.
'dreadfully' indicates anagramming the letters.
'a'+'green'='agreen'
'agreen' with letters rearranged gives 'ENRAGE'.
(Other definitions for enrage that I've seen before include "Make wild" , "Greatly anger" , "Make mad" , "Greatly annoy" , "Incense" .)