Spirit of a guerilla leader with an army (5)
I believe the answer is:
ghost
'spirit' is the definition.
(spirit can mean a ghost)
'guerilla leader with an army' is the wordplay.
'leader' indicates taking the first letters.
'with' means one lot of letters go next to another.
'an army' becomes 'host' (host is a kind of army).
The first letter of 'guerilla' is 'g'.
'g'+'host'='GHOST'
'of a' acts as a link.
(Other definitions for ghost that I've seen before include "Spirit of the dead" , "give it up" , "Apparition; faint trace" , "Writer who gives the credit of authorship to someone else" , "Spirit, spectre" .)