Green’s Dictionary of Slang

the buttoned fly of a man’s trousers.

1969		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.
1979		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].