Building has heavy metal casing on hollow fascia (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')
'heavy metal casing on hollow fascia' is the wordplay.
'heavy metal' becomes 'Pb' (Pb is the atomic symbol for lead).
'casing' means one lot of letters goes inside another (I've seen this in other clues).
'on' becomes 're' ('on' can be mean 'with reference to').
'hollow' says to hollow out the word (remove centre letters) (the word is hollowed out).
'fascia' with its centre removed is 'fa'.
're'+'fa'='refa'
'pb' placed around 'refa' is 'PREFAB'.
'has' acts as a link.
(Other definitions for prefab that I've seen before include "Building assembled from pre-manufactured parts, in short" , "Type of house" , "Manufactured in standard sizes and assembled elsewhere, in short" , "Ready-made building" , "British inter-war temporary housing, popularly called" .)