Dispassionate about a Latin mass (4)
I believe the answer is:
calm
'dispassionate' is the definition.
(thesaurus)
'about a latin mass' is the wordplay.
'about' becomes 'c' (circa).
'latin' becomes 'l'.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'c'+'a'+'l'+'m'='CALM'
(Other definitions for calm that I've seen before include "Pacific" , "Peace, tranquillity" , "Composure or serenity" , "Peaceful, undisturbed" , "Imperturbable, soothe" .)