Overturned coach on green causing upset (7)
I believe the answer is:
subvert
'upset' is the definition.
(I've seen this before)
'overturned coach on green' is the wordplay.
'overturned' says the letters should be written backwards.
'coach' becomes 'bus' (synonyms).
'on green' becomes 'vert' (I've seen this before. I am not sure about the 'on' bit.).
'bus' reversed gives 'sub'.
'sub'+'vert'='SUBVERT'
'causing' is the link.
(Other definitions for subvert that I've seen before include "Cause the downfall of; undermine" , "Undermine (someone's authority)" , "Sabotage" , "Morally corrupt" , "Overthrow - corrupt" .)