Drinking a lot of beer, mostly, got upset about that (6)
I believe the answer is:
toping
'drinking' is the definition.
('tope' can be a synonym of 'drink')
'a lot of beer mostly got upset about that' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'beer' becomes 'pint' (beer is typically served by the pint).
'mostly' becomes 'got' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'got upset' shows that the letters should be reversed in order (I've seen 'getting upset' mean this).
'about that' means one lot of letters goes inside another.
'got' in reverse letter order is 'tog'.
'pint' with its last letter removed is 'pin'.
'pin' inserted within 'tog' is 'TOPING'.
Can you help me to learn more?
(Other definitions for toping that I've seen before include "going on regular binges" , "boozing" .)