A battle we need to fight (3,2,4)
I believe the answer is:
war on want
'fight' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'a battle we need' is the wordplay.
'a battle' becomes 'war' (I've seen this in other clues).
'we' becomes 'on' (I am not sure about this - if you are sure you should believe this answer much more).
'need' becomes 'want' (want is a kind of need).
'war'+'on'+'want'='WAR ON WANT'
'to' acts as a link.
Can you help me to learn more?
(Another definition for war on want that I've seen is " charity".)