Irritate at plant (6)
I believe the answer is:
nettle
I believe this clue is a double definition.
'irritate' is the first definition.
('nettle' can be a synonym of 'irritate')
'plant' is the second definition.
(nettle is a kind of plant)
'at' is the link.
(Other definitions for nettle that I've seen before include "Stinging plant; annoy" , "Clasp it firmly and it will not sting" , "anger?" , "sample of herbal tea?" , "Wild plant - annoy" .)