Shock when leader of republic enters battle (6)
I believe the answer is:
fright
'shock' is the definition.
(I know that shock is a kind of alarm)
'leader of republic enters battle' is the wordplay.
'leader of' suggests taking the first letters.
'enters' means one lot of letters goes inside another.
'battle' becomes 'fight' (fight is a kind of battle).
The first letter of 'republic' is 'r'.
'r' put into 'fight' is 'FRIGHT'.
'when' acts as a link.
(Other definitions for fright that I've seen before include "Start" , "Sudden feeling of fear" , "A scare or shock" , "Panic" , "Horror" .)