The most common and most likely answer to this crossword clue is the 8 letter word BOOZE-UP
I've seen this crossword clue in The Guardian
B
O
O
Z
E
-
U
P
Buddy explains!
The straight part
We know that "a wasted evening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a wasted evening = booze-up.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BOOZE-UP mean?
BOOZE-UP noun- A session of heavy drinking.