21 down
Most
rewarding
transposing
soprano
flute
line
(7)
Most rewarding transposing soprano flute line (7)
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The definition is at the beginning of the clue.
How the clue works
The definition is most rewarding. The word "transposing" signals an anagram, and what you're rearranging is S (soprano) plus FLUTE plus L (line) — mix those seven letters together and you get your answer.