Fear fathead correct (6)
I believe the answer is:
fright
'fear' is the definition.
(I know that fear can be written as fright)
'fathead correct' is the wordplay.
'fathead' becomes 'f' (head letter of 'fat').
'correct' becomes 'right' (right can mean correct).
'f'+'right'='FRIGHT'
(Other definitions for fright that I've seen before include "Terror" , "Alarming event" , "Shock, sudden alarm" , "Start" , "Grotesque" .)