You round soldiers up as an adversary (5)
I believe the answer is:
enemy
'an adversary' is the definition.
(thesaurus)
'you round soldiers up' is the wordplay.
'you' becomes 'ye'.
'round' indicates putting letters inside.
'soldiers' becomes 'men' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ye' enclosing 'men' is 'ymene'.
'ymene' in reverse letter order is 'ENEMY'.
'as' acts as a link.
(Other definitions for enemy that I've seen before include "one in opposition" , "Time" , "Rival" , "Armed adversary" , "other side (like 16)" .)