Bristle on barley (3)
I believe the answer is:
awn
'bristle on barley' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for awn that I've seen before include "Bristle on grass spikelet" , "Beard of cereal" , "Bristles" , "Bristle on cereal spike" , "Stiff bristle on grasses" .)