Failing to ignore large part of church (4)
I believe the answer is:
apse
'part of church' is the definition.
(part of a church building)
'failing to ignore large' is the wordplay.
'failing' becomes 'lapse' (I've seen this before).
'to ignore' suggests deleting specific letters.
'large' becomes 'L' (eg in clothes sizes).
'lapse' with 'l' removed is 'APSE'.
(Other definitions for apse that I've seen before include "Peas (anag) - found in church" , "part of cathedral" , "Recess with domed rof in a church" , "Peas (anag) -- part of a church" , "room for an altar" .)