Inane, is awfully stupid (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'inane is awfully' is the wordplay.
'awfully' indicates an anagram.
'inane'+'is'='inaneis'
'inaneis' is an anagram of 'ASININE'.
(Other definitions for asinine that I've seen before include "Senseless" , "Silly like a donkey" , "Extremely stupid or foolish" , "Stupid (as a donkey?)" , "Really silly, ridiculous" .)