Type of building (6)
(Other definitions for prefab that I've seen before include "Building assembled from manufactured parts, in short" , "Building up, quickly" , "Cheap building" , "type of accommodation" , "British inter-war temporary housing, popularly called" .)
'type of building' is the definition.
This is all the clue.
(Other definitions for office block that I've seen before include "commercial property" , "Large work building" , "Commercial building" , "Business building" .)