Being so one may react rashly (8)
I believe the answer is:
allergic
'being so' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for allergic that I've seen before include "Hypersensitive to something" , "Vulnerable or sensitive to, perhaps pollen" , "acting defensively" , "Reacting (med.)" , "Having an adverse reaction (to something)" .)