Help to support Republican attack (4)
I believe the answer is:
raid
'attack' is the definition.
(raid is a kind of attack)
'help to support republican' is the wordplay.
'help' becomes 'aid' (synonyms).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'republican' becomes 'r' (abbreviation).
'aid' put after 'r' is 'RAID'.
(Other definitions for raid that I've seen before include "Rifle" , "Stock Exchange activity" , "Enter a territory and take spoils" , "A sudden attack or incursion" , "Invasion" .)