Vehicle returned at eleven (4)
I believe the answer is:
'vehicle' is the definition.
(I know that taxi is a type of motor car)
'returned at eleven' is the wordplay.
'returned' says the letters should be written backwards.
'eleven' becomes 'xi' (Roman numerals).
'at' reversed gives 'ta'.
(Other definitions for taxi that I've seen before include "Handgun" , "Part of fleet" , "Prepare to take off" , "Hired vehicle" , "Vehicle with a meter" .)