Enormous heads of neoclassical icons found during attic conversion (7)
I believe the answer is:
titanic
'enormous' is the definition.
(thesaurus)
'heads of neoclassical icons found during attic conversion' is the wordplay.
'heads of' says to take the initial letters.
'found during' means one lot of letters goes inside another.
'conversion' indicates an anagram.
The initial letters of 'neoclassical icons' is 'ni'.
'attic' anagrammed gives 'titac'.
'ni' placed inside 'titac' is 'TITANIC'.
(Other definitions for titanic that I've seen before include "Ill-fated vessel" , "Huge - disaster struck this ship" , "Exceptionally powerful" , "Huge, like doomed liner that struck iceberg" , "Very big and powerful" .)