Prayer used, gain converts (5,3)
I believe the answer is:
agnus dei
'prayer' is the definition.
(Agnus Dei is an example)
'used gain converts' is the wordplay.
'converts' is an anagram indicator.
'used'+'gain'='usedgain'
'usedgain' anagrammed gives 'AGNUS DEI'.
(Other definitions for agnus dei that I've seen before include "religious piece" , "Latin liturgical prayer, Lamb of God" , "part in requiem" , "Christian emblem" , "Part of Latin Mass" .)