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 can't see how they can define each other.
'the heat switched on' is the wordplay.
'switched' becomes 'sh' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'heat' put into 'sh' is 'SHEATH'.
'has' acts as a link.
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" .)