Attic in building is huge (7)
I believe the answer is:
titanic
'huge' is the definition.
(thesaurus)
'attic in building' is the wordplay.
'building' indicates anagramming the letters.
'attic'+'in'='atticin'
'atticin' is an anagram of 'TITANIC'.
'is' acts as a link.
(Other definitions for titanic that I've seen before include "Of great size, like a sunken ship" , "Gigantic like a famous doomed liner" , "Of exceptional strength or size" , "Giant" , "Twenty one [SHIP]" .)