Building quickly assembled before cracking (6)
I believe the answer is:
prefab
'building' is the definition.
(I have seen 'Temporary building ' mean 'prefab' so perhaps 'building' could also mean 'prefab')
'before cracking' is the wordplay.
'before' becomes 'pre'.
'cracking' becomes 'fab' (I can't explain this - if you can you should give a lot more credence to this answer).
'pre'+'fab'='PREFAB'
'quickly assembled' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for prefab that I've seen before include "home -- temporary one?" , "Ready-made house" , "Building assembled on site" , "Cheap building" , "Easily-assembled 1940s' home" .)