Corporation backed energy that makes no noise (4)
I believe the answer is:
mute
'that makes no noise' is the definition.
(as in muting sound)
'corporation backed energy' is the wordplay.
'corporation' becomes 'tum' ('corporation' can humorously mean belly fat).
'backed' says the letters should be written in reverse.
'energy' becomes 'e' (physics symbol as in E=mcu).
'tum' in reverse letter order is 'mut'.
'mut'+'e'='MUTE'
(Other definitions for mute that I've seen before include "Silencer" , "Silent, not talking" , "Device for softening musical sound" , "One refuses to answer" , "Not pronounced" .)