It's in a church letter (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is a kind of letter)
'it's in a church' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues**).
'in' is an insertion indicator.
'church' becomes 'ch' (common abbreviation).
'a'+'ch'='ach'
'it' inserted inside 'ach' is 'AITCH'.
(Other definitions for aitch that I've seen before include "first bit of hymn?" , "The letter h in full" , "Eighth letter of the English alphabet" , "bone" , "Aspirated letter" .)