Closed non-believer holding Mass (8)
I believe the answer is:
hermetic
'closed' is the definition.
(thesaurus)
'non-believer holding mass' is the wordplay.
'non-believer' becomes 'heretic' (I've seen this before).
'holding' means one lot of letters goes inside another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'heretic' placed around 'm' is 'HERMETIC'.
(Other definitions for hermetic that I've seen before include "Completely sealed" , "Sealed so as to be airtight" .)