Happy couple cracking quickly (5)
I believe the answer is:
apace
'quickly' is the definition.
(I know that quickly can be written as apace)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for apace that I've seen before include "(Move on) swiftly" , "flat out" , "At a rapid rate" , "Quick" , "at high speed" .)