Cleric, head of chapter soon (5)
I believe the answer is:
canon
'cleric' is the definition.
(I've seen this before)
'head of chapter soon' is the wordplay.
'head of' indicates taking the first letters.
'soon' becomes 'anon' (synonyms).
The first letter of 'chapter' is 'c'.
'c'+'anon'='CANON'
(Other definitions for canon that I've seen before include "Senior clergyman" , "Standard" , "collected works" , "Such literary works are genuine" , "Member of the clergy" .)