Some cannibals active in wood (5)
I believe the answer is:
balsa
'wood' is the definition.
(type of wood)
'some cannibals active in' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'BALSA' is hidden within 'cannibals active in'.
(Other definitions for balsa that I've seen before include "South American tree" , "Tree yielding light wood" , "Very lightweight wood from tropical tree" , "Strong lightweight wood" , "Very light timber" .)