Of proven ominousness, it may be deadly (5)
I believe the answer is:
venom
'be deadly' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'of proven ominousness it' is the wordplay.
'of' indicates the answer is hidden within the clue.
'VENOM' is hidden within 'proven ominousness it'.
'may' is the link.
Can you help me to learn more?
(Other definitions for venom that I've seen before include "Snake's poison" , "Virulence" , "Snake poison" , "Bitter malice" , "Toxic substance" .)