Negative reaction about small mark (9)
I believe the answer is:
backslash
'mark' is the definition.
(I've seen this before)
'negative reaction about small' is the wordplay.
'negative reaction' becomes 'backlash'.
'about' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'backlash' enclosing 's' is 'BACKSLASH'.
(Other definitions for backslash that I've seen before include "character" , "line on computer screen" , "//www.crosswordgiant.com/crossword-clue/1252911/" , "One leaning to the left" , "Mark" .)