Reaction is academic by king in clash (8)
I believe the answer is:
backlash
'reaction' is the definition.
(I know that backlash is a type of reaction)
'academic by king in clash' is the wordplay.
'academic' becomes 'ba'.
'by' is a charade indicator (letters next to each other).
'king' becomes 'K' (abbreviation for king in chess).
'in' indicates putting letters inside.
'k' put into 'clash' is 'cklash'.
'ba'+'cklash'='BACKLASH'
'is' is the link.
(Other definitions for backlash that I've seen before include "Repercussion" , "Strong negative reaction to a development" , "Violent reaction to an event - using whip?" , "Hostile reaction" , "Sudden violent reaction to an action or situation" .)