Spreading unction's aim, so unctuous (13)
I believe the answer is:
sanctimonious
'unctuous' is the definition.
(thesaurus)
'spreading unction's aim so' is the wordplay.
'spreading' indicates an anagram.
'unctions'+'aim'+'so'='unctionsaimso'
'unctionsaimso' anagrammed gives 'SANCTIMONIOUS'.
(Other definitions for sanctimonious that I've seen before include "Hypocritically holy" , "Hypocritically pious" , "Canting" , "Self-righteous" , "with a holier-than-thou attitude" .)