godan verb (-su), transitive verb
write a new work; write specially for
Used when an author creates an original piece (novel, play, song, script, etc.) specifically for a particular occasion, publication, or performance, rather than reusing existing material.
この雑誌のために、人気作家が短編小説を書き下ろした。
A popular author wrote a new short story specially for this magazine.
彼女は映画のために主題歌を書き下ろした。
She wrote a theme song specifically for the movie.
今回の公演では、劇作家が新作を書き下ろす予定だ。
For this performance, the playwright plans to write a new work.