Failure a king put on instrument (9)
I believe the answer is:
bombardon
'instrument' is the definition.
(bombardon is a kind of instrument)
'failure a king put on' is the wordplay.
'failure' becomes 'bomb' (bomb is a kind of failure).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'put on' becomes 'don' (as in donning or putting on an item of clothing).
'bomb'+'a'+'r'+'don'='BOMBARDON'
(Other definitions for bombardon that I've seen before include "Stop" , "instrument" .)