On a mystic bent, adopting new expression of holiness (10)
I believe the answer is:
sanctimony
'expression of holiness' is the definition.
(acting morally superior or holy)
'on a mystic bent adopting new' is the wordplay.
'bent' indicates an anagram.
'adopting' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'on'+'a'+'mystic'='onamystic'
'onamystic' with letters rearranged gives 'sactimony'.
'sactimony' placed around 'n' is 'SANCTIMONY'.
(Other definitions for sanctimony that I've seen before include "Simulated holiness" , "Pious hypocrisy" , "false piety!" .)