What does the cryptic crossword clue "I sing with lads when playing what might be written in score (9)" mean?

The definition is "what might be written in score". The wordplay uses an anagram. anagram of I SING LADS = GLISSANDI The answer is GLISSANDI.

I sing with lads when playing what might be written in score (9)

Guardian Cryptic #29096 | 5 Down | 2023-06-14 flower