Answer, react (7)
I believe the answer is:
respond
I believe this clue is a double definition.
'answer' is the first definition.
(I know that answer is a more specific form of respond)
'react' is the second definition.
(I know that respond is a more specific form of react)
(Other definitions for respond that I've seen before include "React (positively) to a stimulus or treatment" , "Come back" , "State in reply" , "Answer back" , "Answer or react" .)