To show off, get up on piano (10)
I believe the answer is:
grandstand
'to show off' is the definition.
(I've seen this before)
'get up on piano' is the wordplay.
'get up' becomes 'stand' (to stand up from a sitting position).
'on' means one lot of letters go next to another.
'piano' becomes 'grand piano' (grand is a kind of piano).
'stand' put after 'grand' is 'GRANDSTAND'.
(Other definitions for grandstand that I've seen before include "Racecourse structure" , "Racecourse building" , "Main structure for watching sporting events" , "Main area for race spectators" , "Show off in America" .)