Church singer in vulgar assurance (9)
I believe the answer is:
certitude
'assurance' is the definition.
(thesaurus)
'church singer in vulgar' is the wordplay.
'church' becomes 'CE' (abbreviation for Church of England).
'singer' becomes 'tit' (type of songbird).
'in' indicates putting letters inside.
'vulgar' becomes 'rude' (I've seen this before).
'tit' going inside 'rude' is 'rtitude'.
'ce'+'rtitude'='CERTITUDE'
(Other definitions for certitude that I've seen before include "Something about which one is sure" , "Feeling of surety" , "One's sure" , "A feeling of conviction, given rectitude" .)