Argue to get a new tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'argue to get a new' is the wordplay.
'argue' becomes 'row' (to row with someone is to argue with them).
'to get' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'row'+'a'+'n'='ROWAN'
(Other definitions for rowan that I've seen before include "ash native to mountain regions" , "Mountain-ash" , "source of berries" , "Tree bearing bright red berries" , "Tree - archbishop" .)