In that case mum has the heat switched on (6)
I believe the answer is:
sheath
'in that case mum' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'has the heat switched on' is the wordplay.
'has' becomes 'sh' (I can't justify this - if you can you should believe this answer much more).
'switched' indicates an anagram (letters switch places).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'heat' with letters rearranged gives 'eath'.
'sh' inserted inside 'eath' is 'SHEATH'.
Can you help me to learn more?
(Other definitions for sheath that I've seen before include "Protective cover" , "Cover protecting" , "Blade-cover" , "Casing for a sword" , "Case for a dagger" .)