As insulated building, possibly of fantastic age, bulldozed (6-6)
I believe the answer is:
double-glazed
'as insulated building' is the definition.
(double-glazed windows help insulate a house)
'of fantastic age bulldozed' is the wordplay.
'of fantastic' is an anagram indicator (I've seen 'fantastic' mean this).
'age'+'bulldozed'='agebulldozed'
'agebulldozed' with letters rearranged gives 'DOUBLE-GLAZED'.
'possibly' acts as a link.
(Other definitions for double-glazed that I've seen before include "Two panes thick" , "With a further set of windowpanes" , "Like many windows" .)