Number attached to note on church clock (6)
I believe the answer is:
notice
'clock' is the definition.
(clock can mean to notice or spot)
'number attached to note on church' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'attached to' means one lot of letters go next to another.
'note' becomes 'ti' (musical note, as in do re mi so fa la ti do).
'on' means one lot of letters go next to another.
'church' becomes 'CE' (abbreviation for Church of England).
'no'+'ti'+'ce'='NOTICE'
(Other definitions for notice that I've seen before include "Advance warning given of resignation" , "Review - poster" , "Spot - review" , "Advance warning of an event" , "Take cognisance of, observe" .)