Noisy scene, mass online chat (4)
I believe the answer is:
zoom
'online chat' is the definition.
(conversation using Zoom software)
'noisy scene mass' is the wordplay.
'noisy scene' becomes 'zoo' (zoo can mean chaotic situation).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'zoo'+'m'='ZOOM'
(Other definitions for zoom that I've seen before include "Change focus smoothly" , "Rush around" , "pelt" , "Move quickly - rise rapidly" , "Quickly shift" .)