Spirits good with armies (6)
I believe the answer is:
ghosts
'spirits' is the definition.
(ghost is a kind of spirit)
'good with armies' is the wordplay.
'good' becomes 'g' (abbreviation).
'with' is a charade indicator (letters next to each other).
'armies' becomes 'hosts' (host is a kind of army).
'g'+'hosts'='GHOSTS'
(Other definitions for ghosts that I've seen before include "Spirits or apparitions" , "Spooks, spectres" , "Apparitions of a dead people" , "Writes for another; Ibsen play" , "Spectres, wraiths" .)