Neither mate even unmoors (3)
I believe the answer is:
nor
'neither' is the definition.
(eg in the phrase 'neither do I')
'even unmoors' is the wordplay.
'even' means one should take alternating letters (even-numbered letters are used).
The alternate letters of 'unmoors' are 'NOR'.
'mate' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nor that I've seen before include "Word paired with "neither"" , "And not; Norman (abbr.)" , "Neither pairing" , "Conjunction meaning 'and not'" , "In addition, not" .)