The old issue of REBMUN? (4-6)

I believe the answer is:

back-number

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

'the old issue' is the definition.
(a back number is an old issue of a magazine)

I can't explain the remainder of the clue.

(Another definition for back-number that I've seen is " Earlier issue".)

I've seen this clue in The Irish Times.

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