Chop up hard wood (4)
I believe the answer is:
hash
'chop up' is the definition.
(hashing is a kind of chopping up)
'hard wood' is the wordplay.
'hard' becomes 'h'.
'wood' becomes 'ash' (I've seen this before**).
'h'+'ash'='HASH'
(Other definitions for hash that I've seen before include "that's smoked a lot" , "Meat dish; mess, jumble" , "Mess, botch" , "Mess; reheated meal" , "Weed" .)