Nature god holding one old instrument (5)
I believe the answer is:
piano
'instrument' is the definition.
(I know that piano is a type of stringed instrument)
'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' (abbreviation).
'pan' enclosing 'i' is 'pian'.
'pian'+'o'='PIANO'
(Other definitions for piano that I've seen before include "Musical instrument - play quietly" , "key holder" , "Clavier" , "Musical instrument - softly, please" , "Softly (mus.)" .)