Composer's time supporting school over in Bonn (8)
I believe the answer is:
schubert
'composer's' is the definition.
(Franz Schubert, 1797-1828)
'time supporting school over in bonn' is the wordplay.
'time' becomes 't' (abbreviation).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'school' becomes 'sch' (common abbreviation).
'over in bonn' becomes 'uber' (I've seen this in another clue).
'sch'+'uber'='schuber'
't' put after 'schuber' is 'SCHUBERT'.
(Other definitions for schubert that I've seen before include "Franz -, Austrian composer (Trout Quintet, etc)" , "Butchers (anag) - composer, d. 1828" , "barman" , "Austrian composer of the Trout piano quintet" , "Franz --, his Unfinished symphony is famous" .)