The most common and most likely answer to this crossword clue is the 10 letter word SIGHT-READ
I've seen this crossword clue in The Times
S
I
G
H
T
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R
E
A
D
Buddy explains!
The straight part
We know that "Must those who don't know the score do so" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case must those who don't know the score do so = sight-read.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 SIGHT-READ mean?
SIGHT-READ verb- To perform a musical piece while reading it for the first time, without rehearsal.