Relative's piano - so new (8)
I believe the answer is:
grandson
'relative's' is the definition.
(grandson is a type of relative)
'piano so new' is the wordplay.
'piano' becomes 'grand piano' (grand piano is a kind of piano).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'grand'+'so'+'n'='GRANDSON'
(Other definitions for grandson that I've seen before include "Male child of one's child" , "Male offspring, third generation (8)" , "Daughter's boy" , "issue for daughter, perhaps" , "Dons rang (anag) - relative" .)