Involving two things (6)
I believe the answer is:
binary
'involving two things' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for binary that I've seen before include "System based on the digits 0 and 1" , "Using the digits 0 and 1" , "A sort of star" , "A system of two stars" , "Base-2 system" .)