It's of negative significance when number four's finished (3)
I believe the answer is:
nor
'it's of negative' is the definition.
I know that 'negative' relates to this answer.
'significance when number four's finished' is the wordplay.
I cannot really understand how this works, but
'number' could be 'no' (abbreviation for 'number') and 'no' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nor that I've seen before include "And not; Norman (abbr.)" , "logic circuit" , "Neither one ... the other" , "Neither pairing" , "Conjunction meaning 'and not'" .)