Neither M nor F (6)
I believe the answer is:
neuter
'neither m nor f' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neuter that I've seen before include "Neither m nor f" , "Neither masculine nor feminine gender" , "Tureen (anag.)" , "Make ineffective; gender" , "Of noun, neither masculine nor feminine" .)