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' says to put letters next to each other (I've seen this in other clues).
'be back' says the letters should be written backwards.
'eleven' becomes 'xi' (Roman numerals).
'at' in reverse letter order is 'ta'.
'ta' put next to '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" .)