Abusive to a hothead, madly rash (5)
I believe the answer is:
harsh
'abusive' is the definition.
(thesaurus)
'hothead madly rash' is the wordplay.
'hothead' becomes 'h' (head letter of 'hot').
'madly' is an anagram indicator.
'rash' with letters rearranged gives 'arsh'.
'h'+'arsh'='HARSH'
'to a' acts as a link.
(Other definitions for harsh that I've seen before include "Rough and disagreeable, maybe of voice" , "Jarring" , "Unpleasantly stern" , "'Severe, rough (5)'" , "Abrasive, rough or unkind" .)