Alarm Republican leaving for battle (5)
I believe the answer is:
fight
'for battle' is the definition.
(synonyms)
'alarm republican leaving' is the wordplay.
'alarm' becomes 'fright' (both can mean fear or worry).
'republican' becomes 'r' (abbreviation).
'leaving' indicates named letters should be taken away.
'fright' with 'r' taken out is 'FIGHT'.
(Other definitions for fight that I've seen before include "Strive - struggle" , "Brawl; argue" , "Attack or engage in combat" , "Contend" , "Spar" .)