Where proverbial cheques are quaintly set for battle (2,3,4)

I believe the answer is:

in the mail

I'm a little stuck... Click here to teach me more about this clue!

If this is the answer I cannot explain how the clue works.

(Other definitions for in the mail that I've seen before include "Posted" , "On its way by post" .)

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app