Ponders about how to react (7)
I believe the answer is:
respond
'how to react' is the definition.
(I know that respond is a more specific form of the action react)
'ponders about' is the wordplay.
'about' indicates an anagram.
'ponders' anagrammed gives 'RESPOND'.
(Other definitions for respond that I've seen before include "Write back" , "Come back" , "Reply - react" , "Answer back" , "Answer or react" .)