Destroy idea and react badly (9)
I believe the answer is:
eradicate
'destroy' is the definition.
(to eradicate is to destroy or eliminate)
'idea and react badly' is the wordplay.
'and' says to put letters next to each other.
'badly' indicates anagramming the letters (letters in the wrong order).
'idea'+'react'='ideareact'
'ideareact' anagrammed gives 'ERADICATE'.
(Other definitions for eradicate that I've seen before include "Exterminate, extinguish" , "Stamp out, remove all traces of" , "Kill" , "Excise" , "Get rid of, eliminate entirely" .)