At a very high temperature (6)
'at a very high temperature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for red hot that I've seen before include "Sexy" , "Very busy" , "At a very high temperature" , "highly emotional" , "Sizzling" .)
(Another definition for redhot that I've seen is " Poker tip may be".)