Small piano (6)
'small piano' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spinet that I've seen before include "Instep (anag)" , "Old keyboard instrument" , "Step in with old harpsichord" , "organ" , "Harpsichord-like keyboard instrument" .)
(Other definitions for baby grand that I've seen before include "message of proud new parents?" , "Type of piano" , "Small, non-upright piano" , "Infant piano?" , "Smaller piano" .)