Wood on motorway becomes pulp (4)
I believe the answer is:
mash
'pulp' is the definition.
(I've seen this in another clue)
'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' put after 'm' is 'MASH'.
'becomes' is the link.
(Other definitions for mash that I've seen before include "(Crush into) pulp" , "Creamed potatoes" , "Pulverise" , "Forces hospital" , "Black comedy film set in Korean War" .)