Relative's piano, a piano that's upright (7)
I believe the answer is:
grandpa
'relative's' is the definition.
(grandpa is a kind of relative)
'piano a piano that's upright' is the wordplay.
'piano' becomes 'grand' (grand piano is a kind of piano).
'piano' becomes 'p'.
'that's upright' says the letters should be written backwards.
'a'+'p'='ap'
'ap' reversed gives 'pa'.
'grand'+'pa'='GRANDPA'
(Other definitions for grandpa that I've seen before include "much older relative?" , "Parent's father" , "senior citizen" , "Relation" , "Veteran" .)