Dark blue vehicle reversing at back of alley (4)
I believe the answer is:
navy
'dark blue' is the definition.
('navy' can be a synonym of 'dark blue')
'vehicle reversing at back of alley' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle**).
'reversing' says the letters should be written backwards.
'at' means one lot of letters go next to another.
'back of' indicates one should take the final letters.
The last letter of 'alley' is 'y'.
'van' back-to-front is 'nav'.
'nav'+'y'='NAVY'
(Other definitions for navy that I've seen before include "Nation's warships" , "Fleet - blue" , "Senior service" , "A dark blue; service" , "Fleet of ships" .)