Crossword Genius

Shoot up in Portobello, for one (8)

Ross

I believe the answer is:

mushroom

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'shoot up' is the definition.
(thesaurus)

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

Can you help me to learn more?

(Other definitions for mushroom that I've seen before include "Grow very fast" , "Grow rapidly" , "explode?" , "increase hugely" , "that may be edible?" .)

I've seen this clue in the Irish Times.
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