Spirit of good army (5)
I believe the answer is:
ghost
'spirit' is the definition.
(spirit can mean a ghost)
'good army' is the wordplay.
'good' becomes 'g' (abbreviation).
'army' becomes 'host' (host is a kind of army).
'g'+'host'='GHOST'
'of' acts as a link.
(Other definitions for ghost that I've seen before include "Writer who gives the credit of authorship to someone else" , "Spirit, spectre" , "One's dying to" , "Pen for another" , "21 [WRAITH]" .)