Involving more than one (6)
I believe the answer is:
plural
'involving more than one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for plural that I've seen before include "Denotes more than one" , "More than one, not singular" , "Composed of more than one" , "Refers to two or more" , "Form of noun or verb in respect of more than one" .)