Spooner's prohibited string as stuff for packagers (9)

I believe the answer is:

cardboard

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'stuff for packagers' is the definition.
(as in cardboard packaging)

I don't understand the remainder of the clue.

(Other definitions for cardboard that I've seen before include "stiff material" , "A stiff thick paper" , "without substance" .)

I've seen this clue in The Times.

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