Right to help in an attack (4)
I believe the answer is:
raid
'an attack' is the definition.
(raid is a kind of attack)
'right to help' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to help' becomes 'aid' ('aid' can be a synonym of 'help').
'r'+'aid'='RAID'
'in' is the link.
(Other definitions for raid that I've seen before include "Search by police without warning" , "A sudden attack or incursion" , "Finch and Bronson starred in . . . . On Entebbe" , "Rapid surprise attack" , "A sudden military attack and withdrawal" .)