This uses charade technique - multiple wordplay elements are combined in sequence to build the final answer.
Look for 'BALL' meaning party, then 'nude hen' loses its outer letters to give middle letters, followed by 'night' with the 'n' (new) removed. These elements combine in order.
Answer: BALL-EIGHT | BALL = 'party', E = middle of 'hen' when made 'nude' (outer letters removed), IGHT = 'night' without 'n' for 'new'. Together: BALL + E + IGHT = BALLEIGHT, which is pool (as in a type of pool ball).