Something to make war on, it's growing (5)
I believe the answer is:
rowan
'growing' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'something to make war on' is the wordplay.
'war on' can be anagrammed to 'ROWAN'.
Though, I am not sure how the anagram is indicated.
'it's' is the link.
Can you help me to learn more?
(Other definitions for rowan that I've seen before include "Sort of ash" , "Canterbury" , "Atkinson or Williams" , "Red-berried tree" , "Tree bearing bright red berries" .)