Variable resistor set as “Hotter” (8)
I believe the answer is:
rheostat
'variable resistor' is the definition.
(synonyms)
'set as hotter' is the wordplay.
'set' indicates an anagram (instruction to set the letters).
'as'+'hotter'='ashotter'
'ashotter' with letters rearranged gives 'RHEOSTAT'.
(Other definitions for rheostat that I've seen before include "Resistor for regulating current" , "Means of controlling resistance" , "Current regulator" , "The roast (anag.); electrical instrument" , "Variable resistor" .)