A tree to make war on? (5)
I believe the answer is:
rowan
'a tree' is the definition.
(rowan is a kind of tree)
'make war on?' is the wordplay.
'make' indicates an anagram.
'war'+'on'='waron'
'waron' with letters rearranged gives 'ROWAN'.
'to' is the link.
(Other definitions for rowan that I've seen before include "source of berries" , "Mountain-ash" , "-- Williams, theologian" , "Tree - archbishop" , "ash native to mountain regions" .)