A piece of straw, not a piece of barley (3)
I believe the answer is:
awn
'piece of barley' is the definition.
'awn' can be an answer for 'piece' (awn is a kind of piece). I'm not sure about the 'of barley' bit.
'a piece of straw not a' is the wordplay.
'a piece of' indicates the answer is hidden within the clue.
'AWN' is hidden within 'straw not a'.
Can you help me to learn more?
(Other definitions for awn that I've seen before include "Beard of plant" , "Bristle-like appendage on grass" , "Bristle on cereal spike" , "Bristle on grass spikelet" , "Stiff bristle on grasses" .)