International trio playing anthem (7)
I believe the answer is:
introit
'anthem' is the definition.
(thesaurus)
'international trio playing' is the wordplay.
'international' becomes 'int'.
'playing' is an anagram indicator.
'trio' anagrammed gives 'roit'.
'int'+'roit'='INTROIT'
(Other definitions for introit that I've seen before include "Church psalm" , "Introductory anthem" , "Anthem sung at beginning of Mass" , "Rot in it for a church entry hymn" , "piece of church music" .)