Agree with artist it’s unrealistic idea (7)
I believe the answer is:
chimera
'unrealistic idea' is the definition.
(chimera can mean an unrealistic idea or fantasy)
'agree with artist' is the wordplay.
'agree' becomes 'chime'.
'with' is a charade indicator (letters next to each other).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'chime'+'ra'='CHIMERA'
'it's' acts as a link.
(Other definitions for chimera that I've seen before include "fabulous creature" , "Creature such as a gryphon" , "Wild creation of mind, or monster of Greek myth" , "Fantastic creation of the mind, like Greek mythical monster" , "Each rim (anag) - foolish fancy" .)