Rolled up joint we're told is undersized (4)

I believe the answer is:

puny

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'undersized' is the definition.
(I've seen this in another clue)

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

(Other definitions for puny that I've seen before include "Paltry" , "Small and weak" , "Feeble, under-sized" , "Small and feeble" , "Weak - unimportant" .)

I've seen this clue in The Independent.

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