Briefly, meaning you must get cracking if you want food (2,1,8)

I believe the answer is:

in a nutshell

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'briefly' is the definition.
(synonyms)

I don't understand the rest of the clue.

(Other definitions for in a nutshell that I've seen before include "essentially" , "where you'd find best part of Brazil" , "Expressed briefly" , "Brazil's location" , "Summed up briefly" .)

I've seen this clue in the Evening Standard.

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