Severe rash turned hot (5)
I believe the answer is:
harsh
'severe' is the definition.
(I've seen this before)
'rash turned hot' is the wordplay.
'turned' indicates anagramming the letters.
'hot' becomes 'h' (abbreviation eg on taps).
'rash' is an anagram of 'hars'.
'hars'+'h'='HARSH'
(Other definitions for harsh that I've seen before include "Draconian" , "Severe - unkind" , "Rough and disagreeable, maybe of voice" , "Grating; severe" , "Unkind or cruel" .)