Jack's vocal part of church (5)
I believe the answer is:
knave
'jack's' is the definition.
(knave can mean a jack playing card)
'vocal part of church' is the wordplay.
'vocal' shows a homophone (sound like).
'part of church' becomes 'nave' ('nave' is part of 'church').
'nave' sounds like 'KNAVE'.
(Other definitions for knave that I've seen before include "Yet he may be a diamond" , "[COURT CARD] , maybe" , "Jack - the cad!" , "A rogue as in playing cards" , "geezer" .)