Side of the church not opened by key (4)
I believe the answer is:
isle
'key' is the definition.
(key can mean a small island)
'side of the church not opened by' is the wordplay.
'side of' becomes 'isl' (I can't justify this - if you can you should give a lot more credence to this answer).
'the church' becomes 'CE' (abbreviation for Church of England).
'not opened' means to remove the first letter.
'by' says to put letters next to each other.
'ce' with its initial letter removed is 'e'.
'isl'+'e'='ISLE'
Can you help me to learn more?
(Other definitions for isle that I've seen before include "Hawaii, perhaps" , "Insular place" , "Little land" , "One surrounded by water" , "This could be key" .)