After a day, concludes they’re allies (7)
I believe the answer is:
friends
'allies' is the definition.
('friend' can be a synonym of 'ally')
'after a day concludes' is the wordplay.
'after' is a charade indicator (letters next to each other).
'a day' becomes 'fri'.
'concludes' becomes 'ends' (I've seen this before).
'fri' put next to 'ends' is 'FRIENDS'.
'they're' acts as a link.
(Other definitions for friends that I've seen before include "Chums" , "Intimates" , "playmates!" , "Intimate acquaintances" , "''..., Romans, countrymen, lend me your ears'' (Julius Caesar)" .)