Occasionally play Greek instrument (4)
I believe the answer is:
lyre
'instrument' is the definition.
(lyre is a kind of instrument)
'occasionally play greek' is the wordplay.
'occasionally' means one should take alternating letters.
'play'+'greek'='playgreek'
The alternate letters of 'playgreek' are 'LYRE'.
(Other definitions for lyre that I've seen before include "Old instrument, like a harp" , "used to produce music" , "Instrument played by Orpheus" , "Type of harp" , "Music-maker" .)