Sort of school for a piano salesman? (4)
I believe the answer is:
prep
'sort of school' is the definition.
'piano salesman?' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'salesman?' becomes 'rep' (sales representative).
'p'+'rep'='PREP'
'for a' is the link.
(Other definitions for prep that I've seen before include "warm-up race" , "Homework (abbr.)" , "Homework (abbrev.)" , "School work done outside lessons" .)