Church function includes mostly pleasant historical record (9)
I believe the answer is:
chronicle
'historical record' is the definition.
(I've seen this before)
'church function includes mostly pleasant' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'function' becomes 'role' ('role' can be a synonym of 'function'**).
'includes' means one lot of letters goes inside another.
'mostly' means to remove the last letter.
'pleasant' becomes 'nice' ('nice' is associated in meaning with 'pleasant').
'nice' with its final letter taken off is 'nic'.
'role' going around 'nic' is 'ronicle'.
'ch'+'ronicle'='CHRONICLE'
(Other definitions for chronicle that I've seen before include "Story" , "journal" , "Record of historic events" , "(Keep an) historical record" , "Historical account" .)