Instrument turning up as a collector's piece (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'turning up as a collector's piece' is the wordplay.
'turning up' says the letters should be written in reverse.
'piece' indicates the answer is hidden within the clue.
'ollec' is hidden within 'as collectors'.
'ollec' reversed gives 'CELLO'.
(Other definitions for cello that I've seen before include "Music-maker" , "Bowed musical instrument" , "Bass stringed instrument" , "Instrument in the violin family" , "Musical instrument like large violin held between legs" .)