Head holds sanctimonious belief (7)
I believe the answer is:
opinion
'belief' is the definition.
(I've seen this before)
'head holds sanctimonious' is the wordplay.
'head' becomes 'onion' (I've seen this before**).
'holds' indicates putting letters inside.
'sanctimonious' becomes 'pi'.
'onion' enclosing 'pi' is 'OPINION'.
(Other definitions for opinion that I've seen before include "Assessment" , "Unproved belief or view" , "Personal belief or judgment" , "Personal belief without proof" , "'Judgment, view (7)'" .)