Bourbon barrel material (3)
I believe the answer is:
oak
'bourbon barrel material' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oak that I've seen before include "Its seed is the acorn" , "Acorn-bearing tree" , "Traditional British tree" , "Hard durable wood of a deciduous tree" , "Strong hardwood" .)