Dirt bus dispersed causes upset (7)
I believe the answer is:
disturb
'upset' is the definition.
(I know that upset can be written as disturb)
'dirt bus dispersed' is the wordplay.
'dispersed' is an anagram indicator.
'dirt'+'bus'='dirtbus'
'dirtbus' anagrammed gives 'DISTURB'.
'causes' acts as a link.
(Other definitions for disturb that I've seen before include "Interrupt; upset" , "Unsettle, discomfit" , "Upset, confuse" , "Ruffle - worry" , "Molest" .)