The most common and most likely answer to this crossword clue is the 6 plus 6 letter word TREBLE TWENTY
I've seen this crossword clue in The Guardian
T
R
E
B
L
E
T
W
E
N
T
Y
Buddy explains!
The straight part
We know that "Maximum score on board/Maximum score on board" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case maximum score on board/maximum score on board = treble twenty.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.