Harmony daughter found on old lady’s piano (7,5)
I believe the answer is:
concert grand
'piano' is the definition.
(concert grand is a kind of piano)
'harmony daughter found on old lady's' is the wordplay.
'harmony' becomes 'concert' (I've seen this in another clue).
'daughter' becomes 'd' (genealogical abbreviation).
'found on' means one lot of letters go next to another.
'old lady' becomes 'gran' (short for grandmother).
'd' put after 'gran' is 'grand'.
'concert'+'grand'='CONCERT GRAND'
(Another definition for concert grand that I've seen is " Large piano".)