Portray pope, possibly, as a killjoy (5-6)
I believe the answer is:
party pooper
'a killjoy' is the definition.
('party pooper' can be a synonym of 'killjoy')
'portray pope possibly' is the wordplay.
'possibly' is an anagram indicator.
'portray'+'pope'='portraypope'
'portraypope' is an anagram of 'PARTY-POOPER'.
'as' is the link.
(Other definitions for party pooper that I've seen before include "Social 1 across" , "Spoilsport" , "One who spoils a social occasion" , "One spoiling others' fun" .)