Spartan hothead, unreasonably rash (5)
I believe the answer is:
harsh
'spartan' is the definition.
(I've seen this in another clue)
'hothead unreasonably rash' is the wordplay.
'hothead' becomes 'h' (head letter of 'hot').
'unreasonably' indicates anagramming the letters.
'rash' with letters rearranged gives 'arsh'.
'h'+'arsh'='HARSH'
(Other definitions for harsh that I've seen before include "Severe and disagreeable" , "Abrasive, rough or unkind" , "Unpleasantly stern" , "Rough and disagreeable, maybe of voice" , "Jarring" .)