Thus piano extended old singer (7)
I believe the answer is:
soprano
'singer' is the definition.
(soprano is a kind of singer)
'thus piano extended old' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'piano' becomes 'p'.
'extended' becomes 'ran' ('run' can be a synonym of 'extend').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'so'+'p'+'ran'+'o'='SOPRANO'
(Other definitions for soprano that I've seen before include "Woman singing" , "High female voice" , "Singer who aims high" , "Highest adult female voice" , "(Singer of) the highest vocal range" .)