Polite gathering still up drinking a bit
I believe the answer is:
tea party
'polite gathering' is the definition.
'tea party' can be an answer for 'gathering' (tea party is a kind of gathering). I am not certain of the 'polite' bit.
'still up drinking a bit' is the wordplay.
'still' becomes 'yet' ('yet' can be a synonym of 'still').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'drinking' means one lot of letters goes inside another.
'a bit' becomes 'apart' (I can't explain this - if you can you should give a lot more credence to this answer).
'yet' reversed gives 'tey'.
'tey' placed around 'apart' is 'TEA-PARTY'.
Can you help me to learn more?
(Other definitions for tea party that I've seen before include "Social occasion in the afternoon" , "American political group" , "US political movement" , "Function Alice attended" , "Sober occasion" .)