Sneaky, frequently hiding piano (6)
I believe the answer is:
softly
'piano' is the definition.
(I know that piano can be written as softly)
'sneaky frequently hiding' is the wordplay.
'sneaky' becomes 'sly' (I've seen this in another clue).
'frequently' becomes 'oft' (oft can mean frequently, eg oft-quoted).
'hiding' is an insertion indicator.
'sly' enclosing 'oft' is 'SOFTLY'.
(Other definitions for softly that I've seen before include "Lightly, quietly" , "Quietly or gently" , "In a quiet way" , "In a yielding manner" , "With a quiet and gentle voice" .)