Crossword Genius

Grime Romeo returned in pen case (9)

Ross

I believe the answer is:

cartridge

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

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

Can you help me to learn more?

(Other definitions for cartridge that I've seen before include "holds a lot of ink" , "stunning charge may be made" , "round" , "Bullet's casing" , "Credit rag (anag)" .)

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