Very large attic in conversion (7)
I believe the answer is:
titanic
'very large' is the definition.
(I've seen this before)
'attic in conversion' is the wordplay.
'conversion' is an anagram indicator.
'attic'+'in'='atticin'
'atticin' is an anagram of 'TITANIC'.
(Other definitions for titanic that I've seen before include "Huge, epic" , "made of metal" , "Of enormous size, like the stricken liner" , "Act in it (anag.)" , "Colossal, liner" .)