Tree left next to building (5)
I believe the answer is:
larch
'tree' is the definition.
(larch is a kind of tree)
'left next to building' is the wordplay.
'left' becomes 'l' (common abbreviation).
'next to' says to put letters next to each other.
'building' becomes 'arch'.
'l'+'arch'='LARCH'
(Other definitions for larch that I've seen before include "Coniferous tree with deciduous bright green needles" , "Supplier of timber" , "Deciduous conifer" , "wood" .)