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