Some poplar, chestnut tree (5)
I believe the answer is:
larch
'tree' is the definition.
(larch tree is a kind of tree)
'some poplar chestnut' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'LARCH' is hidden in the letters of 'poplar chestnut'.
(Other definitions for larch that I've seen before include "wood" , "Coniferous tree with deciduous bright green needles" , "Supplier of timber" , "Deciduous conifer" .)