Idiots read his adult novel (8)
I believe the answer is:
airheads
'idiots' is the definition.
(I've seen this before)
'read his adult novel' is the wordplay.
'adult' becomes 'a' (abbreviation).
'novel' is an anagram indicator (letters in a new or novel order).
'read'+'his'+'a'='readhisa'
'readhisa' with letters rearranged gives 'AIRHEADS'.
(Other definitions for airheads that I've seen before include "Idiots" , "Feather-brains" , "military bases" , "Dopes" , "unthinking people" .)