Dismiss a fathead with anger (4)
I believe the answer is:
fire
'dismiss' is the definition.
(I know that dismiss can be written as fire)
'a fathead with anger' is the wordplay.
'a fathead' becomes 'f' (head letter of 'fat').
'with' says to put letters next to each other.
'anger' becomes 'ire' (ire is intense rage).
'f'+'ire'='FIRE'
(Other definitions for fire that I've seen before include "Manually operated device for dealing with flames" , "Dismiss; bake in a kiln" , "Let off" , "... or combustion" , "Blaze - sack" .)