Fleet vehicle returning at end of day (4)
I believe the answer is:
navy
'fleet' is the definition.
(thesaurus)
'vehicle returning at end of day' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle**).
'returning' says the letters should be written in reverse.
'at end of' suggests the final letters.
The last letter of 'day' is 'y'.
'van' in reverse letter order is 'nav'.
'nav'+'y'='NAVY'
(Other definitions for navy that I've seen before include "military arm" , "Senior service" , "A dark blue; service" , "Marine force" , "Colour of the fleet" .)