Negative reaction when adding second typographical symbol (9)
I believe the answer is:
backslash
'typographical symbol' is the definition.
(I know that backslash is a typographical symbol)
'negative reaction when adding second' is the wordplay.
I cannot quite see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is present in the answer.
'second' could be 'back' (to second a motion is to back it) and 'back' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for backslash that I've seen before include "One leaning to the left" , "Mark" , "line on computer screen" , "//www.crosswordgiant.com/crossword-clue/1252911/" , "character" .)