Pain, performing on old instrument (5)
I believe the answer is:
piano
'instrument' is the definition.
(musical instrument)
'pain performing on old' is the wordplay.
'performing' is an anagram indicator (the letters move around or 'perform').
'on' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pain' anagrammed gives 'pian'.
'pian'+'o'='PIANO'
(Other definitions for piano that I've seen before include "Keyboard instrument" , "Stringed instrument - to be played quietly" , "quiet passage" , "Softly (mus.)" , "Musical instrument - softly, please" .)