Irritate with a hypodermic (6)
I believe the answer is:
needle
'irritate' is the definition.
(I know that needle is a more specific form of beset)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for needle that I've seen before include "Sewer's implement" , "pinnacle of rock" , "Badger with sewing implement" , "Rile" , "one of many on Christmas tree" .)