The most common and most likely answer to this crossword clue is the 8 letter word TIEBREAK
I've seen this crossword clue in The Guardian
T
I
E
B
R
E
A
K
Buddy explains!
The straight part
We know that "way to end a game of tennis" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case way to end a game of tennis = tiebreak.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 TIEBREAK mean?
TIEBREAK noun- A tiebreaker, a game or an extension to a game played to resolve a tied score.