The cab will be back at eleven (4)
I believe the answer is:
taxi
'the cab' is the definition.
(taxi cab)
'will be back at eleven' is the wordplay.
'will' means one lot of letters go next to another (I've seen this in other clues).
'be back' shows that the letters should be reversed in order.
'eleven' becomes 'xi' (Roman numerals).
'at' in reverse letter order is 'ta'.
'ta'+'xi' is 'TAXI'.
(Other definitions for taxi that I've seen before include "(Of a plane) move slowly on the ground" , "Public service vehicle" , "Vehicle that plies for hire" , "Motor cab" , "Prepare to fly" .)