Fancy area at the back of Big Ben perhaps (7)
I believe the answer is:
chimera
'fancy' is the definition.
(chimera can mean a fancy or delusion)
'area at the back of big ben perhaps' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'at the back of' means one lot of letters go next to another.
'big ben perhaps' becomes 'chimer' (Big Ben is a famous bell - bells chime).
'a' after 'chimer' is 'CHIMERA'.
(Other definitions for chimera that I've seen before include "Each rim (anag) - foolish fancy" , "Illusory achievement; mythical beast" , "eaders of radio appear after jingle" , "Race him (anag.)" , "Lion-headed monster" .)