Shadow army under general's leadership (5)
I believe the answer is:
ghost
'shadow' is the definition.
(ghost is a kind of shadow)
'army under general's leadership' is the wordplay.
'army' becomes 'host' (host is a kind of army).
'under' says to put letters next to each other.
'leadership' says to take the initial letters.
The initial letter of 'generals' is 'g'.
'host' put after 'g' is 'GHOST'.
(Other definitions for ghost that I've seen before include "A spirit from the beyond" , "Write for another" , "Visible manifestation of a dead person" , "Spirit of the dead" , "author for someone" .)