Confined prior to Green Street Festival (9)
I believe the answer is:
pentecost
'festival' is the definition.
(I've seen this before)
'confined prior to green street' is the wordplay.
'confined' becomes 'pent' (similar in meaning).
'prior to' means one lot of letters go next to another.
'green' becomes 'eco' (abbreviation for ecological).
'street' becomes 'st' (abbreviation).
'pent'+'eco'+'st'='PENTECOST'
(Other definitions for pentecost that I've seen before include "Whitsuntide" , "Christian festival on Whit Sunday" .)