Upsets conductor on back of bus (7)
I believe the answer is:
rattles
'upsets' is the definition.
(I know this)
'conductor on back of bus' is the wordplay.
'conductor' becomes 'Rattle' (Rattle is an orchestra conductor).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'back of' says to take the final letters.
The final letter of 'bus' is 's'.
'rattle'+'s'='RATTLES'
(Other definitions for rattles that I've seen before include "Noisy toys for babies" , "Starlet (anag)" .)