Mass support for proper monarchy (5)
I believe the answer is:
realm
'monarchy' is the definition.
(I know that realm is a type of monarchy)
'mass support for proper' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'support for' says to put letters next to each other (in a down clue, some letters 'support' others).
'proper' becomes 'real' (similar in meaning).
'm' after 'real' is 'REALM'.
(Other definitions for realm that I've seen before include "the UK, perhaps" , "field of study" , "Lamer (anag.)" , "Kingdom could be lamer" , "region" .)