To line, add a new tree (5)
I believe the answer is:
rowan
'tree' is the definition.
(type of tree)
'to line add a new' is the wordplay.
'to line' becomes 'row' (a row or line of things).
'add' is a charade indicator (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" , "Canterbury" , "ash native to mountain regions" , "The mountain ash tree" , "Man" .)