The most common and most likely answer to this crossword clue is the 4 letter word SKOL
I've seen this crossword clue in The Guardian
S
K
O
L
More clues leading to the result SKOL
Buddy explains!

The straight part
We know that "cheers!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cheers! = skol.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 SKOL mean?
SKOL verb- To down (a drink).
- (originally and chiefly in Scotland) A drinking-toast; cheers.