Flog a tree (5)
I believe the answer is:
birch
I believe this clue is a double definition.
'flog' is the first definition.
(I know that birch is a more specific form of flog)
'a tree' is the second definition.
(I know that birch is a type of tree)
(Other definitions for birch that I've seen before include "See 1" , "Flog" , "Type of tree" , "Silver-white barked tree" , "Hardy tree" .)