Rustic piper 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)
'rustic piper holding one old' is the wordplay.
'rustic piper' becomes 'Pan' (pipe-playing Greek god).
'holding' means one lot of letters goes inside another.
'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 "entertain with this softly" , "In music, soflty" , "Musical instrument - play quietly" , "Franz Liszt's instrument" , "key holder" .)