Shielded by poplar, chestnut tree (5)
I believe the answer is:
larch
'tree' is the definition.
(larch tree is a kind of tree)
'shielded by poplar chestnut' is the wordplay.
'shielded by' says the answer is hidden in the clue (the clue shields the letters of the answer).
'LARCH' can be found hidden inside 'poplar chestnut'.
(Other definitions for larch that I've seen before include "Supplier of timber" , "wood" , "Coniferous tree with deciduous bright green needles" , "Deciduous conifer" .)