Those present two hours before midnight at a ball (10)
I believe the answer is:
attendance
'those present' is the definition.
(I've seen this before)
'two hours before midnight at a ball' is the wordplay.
'two hours before midnight' becomes 'tten' (I am not sure about this - if you are sure you should believe this answer much more).
'at' says to put letters next to each other (I've seen this in other clues).
'ball' becomes 'dance' (ball can mean an event for dancing).
'tten' after 'a' is 'atten'.
'atten'+'dance'='ATTENDANCE'
Can you help me to learn more?
(Other definitions for attendance that I've seen before include "Presence; gate" , "Number present" , "Number at gathering" , "all that turned up" , "Turnout" .)