Instrument set a pot on fire (10)
I believe the answer is:
pianoforte
'instrument' is the definition.
(pianoforte is a kind of instrument)
'set a pot on fire' is the wordplay.
'set' indicates an anagram (instruction to set the letters).
'a'+'pot'+'on'+'fire'='apotonfire'
'apotonfire' is an anagram of 'PIANOFORTE'.
(Other definitions for pianoforte that I've seen before include "might be grand" , "Joanna" , "Keyboard instrument" .)