Ready-made building (6)
I believe the answer is:
prefab
'ready-made building' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for prefab that I've seen before include "It's readily put together" , "Building easy to assemble" , "Building made for easy assembly (colloq.)" , "Factory-made building" , "Building assembled from manufactured parts, in short" .)