Relative with piano, so nice initially (8)
I believe the answer is:
grandson
'relative' is the definition.
(grandson is a type of relative)
'piano so nice initially' is the wordplay.
'piano' becomes 'grand piano' (grand piano is a kind of piano).
'initially' indicates taking the first letters.
The first letter of 'nice' is 'n'.
'grand'+'so'+'n'='GRANDSON'
'with' is the link.
(Other definitions for grandson that I've seen before include "Daughter's boy" , "Male child of one's child" , "Young male relative" , "Male offspring, third generation (8)" , "Junior relative" .)