Involving two parties (9)
'involving two parties' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bilateral that I've seen before include "involving two sides" , "Undertaken by two parties" , "involving more than one party?" , "Two-sided" , "Involving two parties" .)
(Another definition for bipartite that I've seen is " Involving two sides".)