After midnight, many phantoms (6)
I believe the answer is:
ghosts
'phantoms' is the definition.
(I know this)
'after midnight many' is the wordplay.
'after' says to put letters next to each other.
'midnight' becomes 'g' (the middle letter in 'night').
'many' becomes 'hosts' (I am not sure about this - if you are sure you should believe this answer much more).
'g' next to 'hosts' is 'GHOSTS'.
Can you help me to learn more?
(Other definitions for ghosts that I've seen before include "Writes for another; Ibsen play" , "Spectres, wraiths" , "Spirits or apparitions" , "Spooks, spectres" , "Apparitions of a dead people" .)