Ceremonial mass held by father (4)
I believe the answer is:
pomp
'ceremonial' is the definition.
(I've seen this before)
'mass held by father' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'held by' is an insertion indicator.
'father' becomes 'pop' (pop is a kind of father).
'm' going into 'pop' is 'POMP'.
(Other definitions for pomp that I've seen before include "Vainglory" , "Great show" , "Grandiosity" , "Ceremonial display" , "Ceremonial elegance and splendour" .)