Cracker put together quickly (5,3)
I believe the answer is:
knock out
I believe this clue is a double definition.
'cracker' is the first definition.
(both can mean something excellent)
'put together quickly' is the second definition.
(as in knocking out a piece of work)
(Other definitions for knock out that I've seen before include "Eliminate" , "Stun - defeat - destroy" , "Render senseless" .)