Respond (to) (5)
(Other definitions for react that I've seen before include "Respond emotionally" , "Hit back" , "Behave in response" , "Change chemically" , "Respond to a stimulus" .)
'respond to' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for answer that I've seen before include "Retort" , "Solve - solution" , "Write back" , "Respond" , "Solution (to problem)" .)