How to give mate a treat at Christmas? (9)
I believe the answer is:
mincemeat
'a treat at christmas?' is the definition.
(mixture in mince pies)
'how to give mate' is the wordplay.
'how' becomes 'mince' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to give' indicates anagramming the letters (I've seen 'giving' mean this (the letters give the letters for another word)).
'mate' with letters rearranged gives 'meat'.
'mince'+'meat'='MINCEMEAT'
Can you help me to learn more?
(Other definitions for mincemeat that I've seen before include "Spiced mixture of raisins, candied peel, apples etc" , "Tart ingredient" , "pie-fillers" , "What did one make of the defeated" , "Christmas pie filling" .)