Thorn found in banks of balsam herb (4)
I believe the answer is:
barb
'thorn' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for barb that I've seen before include "Backward-facing part of hook" , "nasty comment" , "Aggressive remark intended to have a telling effect" , "Stage whispers" , "criticism" .)