Green’s Dictionary of Slang

spare n.1

[dial. spare, a slit in the front of a garment]

the buttoned fly of a man’s trousers.

[Ire]P. Boyle All Looks Yellow to the Jaundiced Eye 55: Button up your spare. Don’t you know full well [...] that exposing your person in public is a criminal offence.
[Ire]D. Marcus ‘Beginnings’ Body and Soul n.p.: Homely bespectacled Mrs Poots [...] ripped open my fly! (I should say ’spare’, for in those days zips were a laughable Yankee eccentricity) [BS].