Very quick, when working out? (8)
I believe the answer is:
cracking
'working out?' is the definition.
(crack can mean to decipher or work out)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cracking that I've seen before include "failing" , "great!" , "Wonderful" , "Excellent; fracturing" , "very fast" .)