Dark blue commercial vehicle overturned on end of motorway (4)
I believe the answer is:
navy
'dark blue' is the definition.
(synonyms)
'commercial vehicle overturned on end of motorway' is the wordplay.
'commercial vehicle' becomes 'van' (I've seen this before).
'overturned' says the letters should be written backwards.
'on' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The last letter of 'motorway' is 'y'.
'van' written backwards gives 'nav'.
'nav'+'y'='NAVY'
(Other definitions for navy that I've seen before include "Fleet of ships" , "A dark blue; service" , "Shade of the marine forces" , "Marine force" , "Armed force" .)