Negative link (3)
I believe the answer is:
nor
'negative link' is the definition.
'nor' can be an answer for 'link' (I've seen this before). I also know that 'negative' relates to this answer.
This is the entire clue.
(Other definitions for nor that I've seen before include "Link word" , "Conjunction meaning 'and not'" , "Neither one ... the other" , "And not; Norman (abbr.)" , "Common conjunction, goes with neither" .)