Crush tree under motorway (4)
I believe the answer is:
mash
'crush' is the definition.
('mash' can be a synonym of 'crush')
'tree under motorway' is the wordplay.
'tree' becomes 'ash' (ash is a kind of tree).
'under' says to put letters next to each other.
'motorway' becomes 'm'.
'ash' put after 'm' is 'MASH'.
(Other definitions for mash that I've seen before include "Pured potato" , "Pummel" , "Forces hospital" , "Soft, pulpy mass" , "Bangers' frequent partner?" .)