Confident Republican in to take action (4)
I believe the answer is:
sure
'confident' is the definition.
(similar in meaning)
'republican in to take action' is the wordplay.
'republican' becomes 'R' (abbreviation in US politics).
'in' indicates putting letters inside.
'to take action' becomes 'sue' (sue can mean to take legal action in court).
'r' inserted within 'sue' is 'SURE'.
(Other definitions for sure that I've seen before include "Convinced" , "Certain, confident" , "The old sewer" , "Completely confident that one is right" , "Agreed" .)