Sally is right to help (4)
I believe the answer is:
raid
'sally' is the definition.
(both can mean a military 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'
'is' is the link.
(Other definitions for raid that I've seen before include "Invade" , "A sudden military attack and withdrawal" , "Rifle" , "Rapid surprise attack" , "Foray, sudden attack" .)