Drier shelf stocking one pen (6,3)

I believe the answer is:

silica gel

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

'drier' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Another definition for silica gel that I've seen is " drier".)

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

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