Good army haunt (5)
I believe the answer is:
ghost
'haunt' is the definition.
('ghost' can be a synonym of 'haunt')
'good army' is the wordplay.
'good' becomes 'g' (abbreviation).
'army' becomes 'host' (I've seen this before).
'g'+'host'='GHOST'
(Other definitions for ghost that I've seen before include "insubstantial presence" , "Author, secretly" , "Supernatural apparition" , "Lay down life for another in this" , "Write for another" .)