Where set of keys may be found by relative opening grave (4,5)
I believe the answer is:
baby grand
'where set of keys may be found' is the definition.
'by relative opening grave' is the wordplay.
'relative' becomes 'gran' (I've seen this in other clues**).
'opening' means one lot of letters goes inside another (some letters open a gap in others).
'grave' becomes 'bad'.
'by'+'gran'='bygran'
'bygran' placed into 'bad' is 'BABY GRAND'.
(Other definitions for baby grand that I've seen before include "Smaller piano" , "message of proud new parents?" , "Type of piano" , "Infant piano?" , "Small, non-upright piano" .)