It's of great ominousness in physics (4)
I believe the answer is:
atom
'in physics' is the definition.
'it's of great ominousness' is the wordplay.
'it's of' indicates the answer is hidden within the clue.
'ATOM' can be found hidden inside 'great ominousness'.
(Other definitions for atom that I've seen before include "Something very small perhaps" , "Scrap" , "Very small object or amount" , "It was once thought to be smallest particle of matter" , "A small but mighty particle" .)