GR translation: With a high slash that Bladdery tries to parry in third

[Mike Jing]

V597.11-17   Krypton stashes Slothrop in a garbage bin and goes looking for
Seaman Bodine. After a number of short, glittering feints, Purfle dodges
in, quick as a fighting-cock. With a high slash that Bladdery tries to
parry in third, Purfle rips through the Commando’s blouse and draws blood.
But when he goes to jump back, it seems thoughtful Bladdery has brought his
combat boot down on the American’s low dress shoe, nailing him where he
stands.

What exactly is "to parry in third"?
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