Poorly earthed – rather high temperature (3,4)
I believe the answer is:
red heat
'rather high temperature' is the definition.
'red heat' can be an answer for 'temperature' (red heat is a kind of temperature). I am not sure about the 'rather high' bit.
'poorly earthed' is the wordplay.
'poorly' is an anagram indicator (a poor spelling).
'earthed' anagrammed gives 'RED HEAT'.
Can you help me to learn more?
(Other definitions for red heat that I've seen before include "High temperature" , "Temperature at which something glows" , "Glowing temperature" , "Fieriness" .)