Confirmed about tree (3)
'tree' is the definition.
(fir tree is a kind of tree)
'confirmed about' is the wordplay.
'about' indicates the answer is hidden within the clue.
'FIR' can be found hidden inside 'confirmed'.
(Other definitions for fir that I've seen before include "Kind of cone-bearing tree", "Scots pine", "Evergreen conifer", "Tree with cones", "Coniferous tree".)