Units forming a reserve (4)
I believe the answer is:
ares
'units' is the definition.
(are is a kind of unit)
'a reserve' is the wordplay.
'reserve' becomes 'res' (I've seen this in other clues).
'a'+'res'='ARES'
'forming' acts as a link.
(Other definitions for ares that I've seen before include "Belligerent god" , "Greek god of war" , "Olympian" , "War-god" , "figure of worship" .)