Wood required by barrel makers (3)
I believe the answer is:
elm
'wood' is the definition.
(elm is a type of tree)
'required by barrel makers' is the wordplay.
'required by' indicates the answer is hidden within the clue (the answer is required to spell part of the clue).
'ELM' is hidden within 'barrel makers'.
(Other definitions for elm that I've seen before include "Forest tree" , "Deciduous tree with rough serrated leaves" , "I branch out" , "Mel's tree" , "Furniture wood" .)