Kit one used in a piano piece (American) (9)
I believe the answer is:
Here is my best explanation:
'kit' is the definition.
(I've seen this before)
'one used in a piano piece american' is the subsidiary indication.
'one' becomes 'a'.
'used in' is an insertion indicator.
'piano' becomes 'p'.
'piece' becomes 'part' ('part' can be a synonym of 'piece'**).
'american' becomes 'us'.
'a' going inside 'appart' is 'apparat'.
(Other definitions for apparatus that I've seen before include "Equipment, gear", "Device with specific functions", "Technical equipment", "Collection of equipment designed for a particular purpose", "Technical kit", "Equipment, device", "Set of equipment serving a specific purpose", "Lab equipment", "Equipment designed for a specific function" and "Machinery".)