Tree line to extend over (5)

I believe the answer is:

larch

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'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" .)

I've seen this clue in The Independent and The Sun.

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