Nature god holding one old instrument to play (5)
I believe the answer is:
piano
'instrument to play' is the definition.
'nature god holding one old' is the wordplay.
'nature god' becomes 'pan' (Pan is a god in Greek mythology).
'holding' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pan' going around 'i' is 'pian'.
'pian'+'o'='PIANO'
(Other definitions for piano that I've seen before include "Quiet (music)" , "In musical notation, this means softly" , "Keyboard instrument" , "quiet passage" , "Musical instrument - softly, please" .)