Military action in Sussex town (6)
I believe the answer is:
battle
I believe this clue is a double definition.
'military action' is the first definition.
(battle is a kind of military action)
'sussex town' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for battle that I've seen before include "Armed conflict" , "Eg, Bosworth, Blenheim" , "Fight, struggle" , "Warfare" , "Major engagement" .)