Upright piano, with arrangement of a prelude in C, right? (13)
I believe the answer is:
perpendicular
'upright' is the definition.
('perpendicular' can be a synonym of 'upright')
'piano with arrangement of a prelude in c right?' is the wordplay.
'piano' becomes 'p' (**).
'with' says to put letters next to each other.
'arrangement of' is an anagram indicator.
'right?' becomes 'r' (common abbreviation).
'a'+'prelude'+'in'+'c'='apreludeinc'
'apreludeinc' is an anagram of 'erpendicula'.
'p'+'erpendicula'+'r'='PERPENDICULAR'
(Other definitions for perpendicular that I've seen before include "Vertical - at 90 degrees" , "Vertical; upright" , "Style of late English Gothic church architecture" , "style of construction" , "Gothic style following 7" .)