Temporary housing not yet marvellous? (6)
I believe the answer is:
'temporary housing' is the definition.
(building that can be disassembled easily)
'not yet marvellous?' is the wordplay.
'not yet' becomes 'pre' (I've seen this in other clues).
'marvellous?' becomes 'fab' (slang abbreviation of fabulous).
(Other definitions for prefab that I've seen before include "House constructed away from site" , "Cheap building" , "It's readily put together" , "Temporary accommodation" , "Building assembled on site" .)