Some cannibals assembling wood (5)
I believe the answer is:
balsa
'wood' is the definition.
(type of wood)
'some cannibals assembling' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'BALSA' can be found hidden inside 'cannibals assembling'.
(Other definitions for balsa that I've seen before include "lightweight boat-builder" , "Tree yielding light wood" , "Tropical American tree" , "Very lightweight wood from tropical tree" , "Wood used by model makers" .)