One kept in receptacle, old instrument (5)
I believe the answer is:
piano
'instrument' is the definition.
(musical instrument)
'one kept in receptacle old' is the wordplay.
'one' becomes 'i' (Roman numeral).
'kept in' means one lot of letters goes inside another.
'receptacle' becomes 'pan' (type of container).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'i' going within 'pan' is 'pian'.
'pian'+'o'='PIANO'
(Other definitions for piano that I've seen before include "Instrument with ivories" , "Soft, softly (music)" , "Opposite of forte" , "Direction in music (1): relatively softly" , "Franz Liszt's instrument" .)