Answer right, at once wrong, I assumed (8)
I believe the answer is:
reaction
'answer' is the definition.
(thesaurus)
'right at once wrong i assumed' is the wordplay.
'right' becomes 'r' (common abbreviation**).
'wrong' indicates an anagram.
'assumed' indicates putting letters inside.
'at'+'once'='atonce'
'atonce' anagrammed gives 'eacton'.
'eacton' placed around 'i' is 'eaction'.
'r'+'eaction'='REACTION'
(Other definitions for reaction that I've seen before include "Into care (anag)" , "counter-revolution" , "In mechanics, every action has an equal and opposite one" , "Chemical change or backlash" , "Feedback" .)