Mistake to ignore leader in church area (4)
I believe the answer is:
apse
'church area' is the definition.
(I've seen this before)
'mistake to ignore leader' is the wordplay.
'mistake' becomes 'lapse' (lapse is a kind of mistake).
'to ignore leader' means to remove the first letter (I've seen 'ignoring leader' mean this).
'lapse' with its initial letter removed is 'APSE'.
'in' is the link.
(Other definitions for apse that I've seen before include "Domed section of a church" , "Recess in church wall" , "room for an altar" , "Offshoot from the choir, nave or aisle in a church" , "Peas (anag) -- part of a church" .)