What does the cryptic crossword clue "Playing our piano, starts to compose concerto for horn (10)" mean?

The definition is "horn". The wordplay uses an anagram. anagram of OUR PIANO CC = CORNUCOPIA; definition: "horn" The answer is CORNUCOPIA.

Playing our piano, starts to compose concerto for horn (10)

Guardian Cryptic #29242 | 13 Across | 2023-12-01 flower