Passage in left part of church (5)
I believe the answer is:
lapse
'passage' is the definition.
(I've seen this before)
'left part of church' is the wordplay.
'left' becomes 'l' (common abbreviation).
'part of church' becomes 'apse' ('apse' is part of 'church').
'l'+'apse'='LAPSE'
'in' is the link.
(Other definitions for lapse that I've seen before include "Slight error due to forgetfulness" , "Fault" , "Temporary failure of judgment or concentration" , "Slip-up" , "Revert (to bad habits, etc)" .)