Briefly, meaning you must get cracking if you want food (2,1,8)
I believe the answer is:
in a nutshell
'briefly' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(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" .)