Arm bone fractured beneath hard wood (8)
I believe the answer is:
hornbeam
'wood' is the definition.
(type of tree)
'arm bone fractured beneath hard' is the wordplay.
'fractured' is an anagram indicator.
'beneath' says to put letters next to each other (in a down clue).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'arm'+'bone'='armbone'
'armbone' anagrammed gives 'ornbeam'.
'ornbeam' put after 'h' is 'HORNBEAM'.
(Other definitions for hornbeam that I've seen before include "is found in wood" , "heavy plant" , "Tree good for hedging" , "Timber" , "Do men abhor this tree?" .)