At midnight, a person has left (4)
I believe the answer is:
gone
'left' is the definition.
(go can mean to leave)
'at midnight a person' is the wordplay.
'at' says to put letters next to each other.
'midnight' becomes 'g' (the middle letter in 'night').
'a person' becomes 'one' (an individual).
'g' next to 'one' is 'GONE'.
'has' is the link.
(Other definitions for gone that I've seen before include "Well in the past" , "Moved - died" , "... saga of 2's civil war" , "Away, departed" , "Departed or dead" .)