Report about mass ceremony (4)
I believe the answer is:
pomp
'ceremony' is the definition.
(I've seen this in another clue)
'report about mass' is the wordplay.
'report' becomes 'pop' ('report' can mean a sudden explosive noise).
'about' is an insertion indicator.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'pop' going around 'm' is 'POMP'.
(Other definitions for pomp that I've seen before include "Ceremony or ostentation" , "Excessive ceremony" , "Vain display" , "Vainglory" , "Ceremony and splendour" .)