Any one of the X-Men (6)
I believe the answer is:
mutant
'any one of the x-men' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mutant that I've seen before include "Genetically-modified" , "badly bred" , "Cell that carries an alteration of a gene" , "A product of genetic change" , "Genetically changed form" .)