Heard Jack in part of church (4)
I believe the answer is:
'part of church' is the definition.
('nave' is part of 'church')
'heard jack' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'jack' becomes 'knave' ('knave' can be a synonym of 'jack').
'knave' is a homophone of 'NAVE'.
'in' is the link.
(Other definitions for nave that I've seen before include "Part of the church sounds like a rogue", "Central part of church", "Part of a church sounds roguish", "Central place in church", "Middle of wheel".)