I ignored caustic, irritable, prickly sort (6)
I believe the answer is:
cactus
'prickly sort' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cactus that I've seen before include "Prickly sort" , "Eg, prickly pear" , "Prickly desert plant" , "Succulent desert plant" , "Drought-resistant plant" .)