Agree on academician's idle fancy (7)
I believe the answer is:
chimera
'idle fancy' is the definition.
(chimera can mean a fancy or delusion)
'agree on academician's' is the wordplay.
'agree' becomes 'chime'.
'on' is a charade indicator (letters next to each other).
'academician' becomes 'RA' (member of the Royal Academy of Arts).
'chime'+'ra'='CHIMERA'
(Other definitions for chimera that I've seen before include "wild dream" , "Mythological monster" , "Wild or impossible idea" , "fake image" , "Wild creation of mind, or monster of Greek myth" .)