Confirmation about tree (3)
I believe the answer is:
fir
'tree' is the definition.
(fir tree is a kind of tree)
'confirmation about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'FIR' is hidden within 'confirmation'.
(Other definitions for fir that I've seen before include "Kind of cone-bearing tree" , "Evergreen conifer" , "Scots pine" , "Tree with cones" , "Evergreen tree for Irish men?" .)