Extremely large attic in for conversion (7)
I believe the answer is:
titanic
'extremely large' is the definition.
(I've seen this before)
'attic in for conversion' is the wordplay.
'for conversion' is an anagram indicator (I've seen this in other clues).
'attic'+'in'='atticin'
'atticin' anagrammed gives 'TITANIC'.
(Other definitions for titanic that I've seen before include "Gigantic like doomed ocean liner" , "Ill-fated vessel" , "Huge, epic" , "Doomed liner found in attic" , "Huge, like doomed liner that struck iceberg" .)