Army departs after triumph (5)
I believe the answer is:
crowd
'army' is the definition.
(I've seen this before)
'departs after triumph' is the wordplay.
'departs' becomes 'd' (abbreviation eg in train timetables).
'after' means one lot of letters go next to another.
'triumph' becomes 'crow' (both can mean to celebrate).
'd' put after 'crow' is 'CROWD'.
(Other definitions for crowd that I've seen before include "See 1" , "Rabble" , "Army" , "Horde" , "Multitude" .)