Acclamation of unusual papal custom (8)
I believe the answer is:
applause
'acclamation' is the definition.
(thesaurus)
'unusual papal custom' is the wordplay.
'unusual' indicates anagramming the letters.
'custom' becomes 'use' (use is a kind of custom).
'papal' is an anagram of 'appla'.
'appla'+'use'='APPLAUSE'
'of' is the link.
(Other definitions for applause that I've seen before include "round, perhaps?" , "Gesture of approval" , "Ovation, acclaim" , "Noisy expression of approbation" , "big hand" .)