Playing our piano, starts to compose concerto for horn (10)
I believe the answer is:
cornucopia
'horn' is the definition.
(I have seen 'Horn of plenty' mean 'cornucopia' so perhaps 'horn' could also mean 'cornucopia')
'playing our piano starts to compose concerto' is the wordplay.
'playing' indicates anagramming the letters.
'starts to' says to take the initial letters.
The initial letters of 'compose concerto' is 'cc'.
'our'+'piano'+'cc'='ourpianocc'
'ourpianocc' with letters rearranged gives 'CORNUCOPIA'.
'for' is the link.
(Other definitions for cornucopia that I've seen before include "Horn of plenty" , "There's plenty here!" , "Great abundance" , "mine" , "Abundant supply of good things" .)