Make war on a tree! (5)
I believe the answer is:
rowan
'a tree' is the definition.
(type of tree)
'make war on' is the wordplay.
'make' is an anagram indicator.
'war'+'on'='waron'
'waron' is an anagram of 'ROWAN'.
(Other definitions for rowan that I've seen before include "Man" , "Mountain ash (at Lambeth!)" , "Tree with orange-red berry-like fruits" , "Mr Atkinson, actor" , "The mountain ash tree" .)