Almost entirely indifferent about hot drink (7)
I believe the answer is:
alcohol
'drink' is the definition.
(I know that alcohol is a type of drink)
'almost entirely indifferent about hot' is the wordplay.
'almost entirely' becomes 'al' (I can't explain this - if you can you should give a lot more credence to this answer).
'indifferent' becomes 'cool' (I've seen this in another clue).
'about' is an insertion indicator.
'hot' becomes 'h' (abbreviation eg on taps).
'cool' enclosing 'h' is 'cohol'.
'al'+'cohol'='ALCOHOL'
Can you help me to learn more?
(Other definitions for alcohol that I've seen before include "It can make one merry" , "John Barleycorn" , "Intoxicant in wine, etc." , "Strong drink" , "The hard stuff" .)