Bitter hothead, wildly rash (5)
I believe the answer is:
harsh
'bitter' is the definition.
(thesaurus)
'hothead wildly rash' is the wordplay.
'hothead' becomes 'h' (head letter of 'hot').
'wildly' is an anagram indicator.
'rash' anagrammed gives 'arsh'.
'h'+'arsh'='HARSH'
(Other definitions for harsh that I've seen before include "Draconian" , "Abrasive, rough or unkind" , "Jarring" , "Punitive" , "Raucous - austere - bitter" .)