Change of direction is a shock (4)
I believe the answer is:
turn
I believe this clue is a double definition.
'change of direction' is the first definition.
(turn is a kind of change of direction)
'shock' is the second definition.
(I've seen this in other clues)
'is a' acts as a link.
(Other definitions for turn that I've seen before include "Revolution by runt" , "fit" , "rotation" , "Shift - wheel - round" , "Swing round" .)