Pass by eastern part of church behind back of chancel (6)
I believe the answer is:
elapse
'pass by' is the definition.
(thesaurus)
'eastern part of church behind back of chancel' is the wordplay.
'eastern' becomes 'e'.
'part of church' becomes 'apse' ('apse' is part of 'church').
'behind' says to put letters next to each other.
'back of' suggests the final letters.
The last letter of 'chancel' is 'l'.
'apse' after 'l' is 'lapse'.
'e'+'lapse'='ELAPSE'
(Other definitions for elapse that I've seen before include "Of time, pass by - please" , "Passing" , "Pass away - asleep (anag)" , "Slip away" , "Please (anag) -- pass away" .)