Possible relief from lumbago, an old issue (4,6)

I believe the answer is:

back number

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

I don't understand how the rest of the clue works.

(Other definitions for back number that I've seen before include "An earlier issue" , "Has-been" , "Old paper" , "copy of magazine" , "not a current issue" .)

I've seen this clue in The Times.

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