Line beside a new tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'line beside a new' is the wordplay.
'line' becomes 'row' (a row or line of things).
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'row'+'a'+'n'='ROWAN'
(Other definitions for rowan that I've seen before include "Red-berried tree" , "Tree bearing bright red berries" , "Tree with orange-red berry-like fruits" , "Sort of ash" , "Tree - archbishop" .)