Habit of a sinner straying between masses (9)
I believe the answer is:
mannerism
'habit' is the definition.
(thesaurus)
'a sinner straying between masses' is the wordplay.
'straying' indicates an anagram.
'between' is an insertion indicator.
'masses' becomes 'mm' (the scientific symbol for mass written twice).
'a'+'sinner'='asinner'
'asinner' is an anagram of 'anneris'.
'anneris' put inside 'mm' is 'MANNERISM'.
'of' acts as a link.
(Other definitions for mannerism that I've seen before include "Distinctive personal way of doing something" , "Idiosyncrasy" , "Habitual gesture" , "Habitual way of behaving" , "Italian art?" .)