A third party profiting from a conflict between two others who are at a standstill.

Profit gained by a third party when two others are locked in a struggle; used to describe taking advantage of a conflict between others.

if two parties fight, a third party will benefit from the struggle; to reap the spoils of another's conflict

literally meaning 'when the snipe and the clam grapple, the fisherman benefits'; used to describe a situation where two parties are locked in a dispute, allowing a third party to step in and take advantage of the situation