Painter of unusual old instrument once (10)
I believe the answer is:
fortepiano
'instrument once' is the definition.
(early form of piano)
'painter of unusual old' is the wordplay.
'unusual' indicates anagramming the letters.
'old' becomes 'o' (abbreviation).
'painter'+'of'='painterof'
'painterof' anagrammed gives 'fortepian'.
'fortepian'+'o'='FORTEPIANO'
(Other definitions for fortepiano that I've seen before include "keyboard instrument" , "Old source of music" .)