Wood on motorway causes crush (4)
I believe the answer is:
mash
'crush' is the definition.
(synonyms)
'wood on motorway' is the wordplay.
'wood' becomes 'ash' (wood from an ash 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 "Pummel" , "Pureed potatoes" , "Soft, pulpy mass" , "Pured potato" , "Pureed potato" .)