Tree line to extend over (5)
I believe the answer is:
larch
'tree' is the definition.
(larch is a kind of tree)
'line to extend over' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'to extend over' becomes 'arch' (I can't justify this - if you can you should give a lot more credence to this answer).
'l'+'arch'='LARCH'
Can you help me to learn more?
(Other definitions for larch that I've seen before include "Coniferous tree with deciduous bright green needles" , "Deciduous conifer" , "wood" , "Supplier of timber" .)