School gets a ten for instrument (3)
I believe the answer is:
sax
'for instrument' is the definition.
(saxophone)
'school gets a ten' is the wordplay.
'school' becomes 'S'.
'gets' says to put letters next to each other.
'ten' becomes 'X' (Roman numeral).
's'+'a'+'x'='SAX'
(Other definitions for sax that I've seen before include "Music-maker" , "Jazz instrument (abbr)" , "Wind instrument (abbr)" .)