Step in to play instrument (6)
I believe the answer is:
spinet
'instrument' is the definition.
(spinet is a historical type of musical instrument)
'step in to play' is the wordplay.
'to play' indicates anagramming the letters.
'step'+'in'='stepin'
'stepin' is an anagram of 'SPINET'.
(Other definitions for spinet that I've seen before include "organ" , "Step in with old harpsichord" , "Instrument resembling a small harpsichord" , "A small harpsichord of old" , "Small 12" .)