Exceptionally cold and definitively negative (7)
I believe the answer is:
subzero
'negative' is the definition.
(I've seen this before)
'exceptionally cold and definitively' is the wordplay.
I cannot really see how this works, but
'cold' could be 'zero' (I've seen this before) and 'zero' is present in the answer.
The remaining letters 'sub' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for subzero that I've seen before include "very cold" , "Colder than freezing" , "Negative" , "perishing" , "Extremely nippy" .)