Reacting rashly perhaps to unusual ill-grace (8)
I believe the answer is:
allergic
'reacting rashly perhaps' is the definition.
(an allergy might cause a rash)
'unusual ill-grace' is the wordplay.
'unusual' indicates an anagram.
'ill'+'grace'='illgrace'
'illgrace' with letters rearranged gives 'ALLERGIC'.
'to' acts as a link.
(Other definitions for allergic that I've seen before include "Responding badly (to something)" , "Being so, one may react rashly" , "Hypersensitive to something" , "Reacting badly (to something)" , "Reacting (med.)" .)