God of war, essentially (4)
I believe the answer is:
ares
'god' is the definition.
(Greek god of war)
'of war essentially' is the wordplay.
'of' indicates a hidden word.
'ARES' is hidden in the letters of 'war essentially'.
(Other definitions for ares that I've seen before include "figure of worship" , "Land measures" , "Olympian" , "War-god" , "Greek god of war" .)