Argue by a new tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'argue by a new' is the wordplay.
'argue' becomes 'row' (to row with someone is to argue with them).
'by' 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 "Mr Atkinson, actor" , "Tree with orange-red berry-like fruits" , "Atkinson or Williams" , "Red-berried tree" , "source of berries" .)