Tree on motorway causes crush (4)
I believe the answer is:
mash
'crush' is the definition.
(synonyms)
'tree on motorway' is the wordplay.
'tree' becomes 'ash' (ash tree is a kind of tree).
'on' means one lot of letters go next to another.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'ash' after 'm' is 'MASH'.
'causes' acts as a link.
(Other definitions for mash that I've seen before include "It goes well with sausages" , "Bangers' frequent partner?" , "Animal feed" , "Black comedy film set in Korean War" , "Brew or infuse (tea)" .)