Report about mass or ceremony (4)
I believe the answer is:
pomp
'ceremony' is the definition.
(thesaurus)
'report about mass' is the wordplay.
'report' becomes 'pop' ('report' can mean a sudden explosive noise).
'about' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'pop' placed around 'm' is 'POMP'.
'or' acts as a link.
(Other definitions for pomp that I've seen before include "Ceremoniousness" , "Ceremony or ostentation" , "Boastful display" , "Stately and splendid display" , "Ceremonial grandeur" .)