Vigour shown in over-vehement reaction (5)
I believe the answer is:
verve
'vigour' is the definition.
(I know that verve is a type of vigour)
'in over-vehement reaction' is the wordplay.
'in' indicates the answer is hidden within the clue.
'VERVE' is hidden within 'over vehement reaction'.
'shown' is the link.
(Other definitions for verve that I've seen before include "zip" , "Gusto; energy" , "Animation" , "Vitality" , "Root tasting of oysters" .)