Callow youth or environmentalist with hooter (9)
I believe the answer is:
greenhorn
'callow youth' is the definition.
(a greenhorn is an inexperienced person)
'environmentalist with hooter' is the wordplay.
'environmentalist' becomes 'green' (member of the Green Party).
'with' is a charade indicator (letters next to each other).
'hooter' becomes 'horn' (instrument making a hooting sound).
'green'+'horn'='GREENHORN'
'or' acts as a link.
(Other definitions for greenhorn that I've seen before include "Tenderfoot" , "Novice" , "Raw, inexperienced youth" , "One learning" , "colt" .)