Stab snake, being impulsive (9)
I believe the answer is:
whirlwind
'being impulsive' is the definition.
(I've seen this before)
'stab snake' is the wordplay.
'stab' becomes 'whirl' (I've seen this in another clue).
'snake' becomes 'wind' (to wind is to snake around).
'whirl'+'wind'='WHIRLWIND'
(Other definitions for whirlwind that I've seen before include "E.g. a dust devil" , "Impulsive" , "(Of a romance) rapidly sweeping you off your feet?" , "Cyclone" , "Energetic person" .)