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' means one lot of letters go next to another.
'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" .)