Halfwit is terribly smug on air (9)
I believe the answer is:
ignoramus
'halfwit' is the definition.
'terribly smug on air' is the wordplay.
'terribly' indicates anagramming the letters.
'smug'+'on'+'air'='smugonair'
'smugonair' with letters rearranged gives 'IGNORAMUS'.
'is' is the link.
(Other definitions for ignoramus that I've seen before include "I wouldn't know" , "Stupid person" , "At the present time" , "A know-nothing" , "one in a state of bliss?" .)