Go off riding after shock (4,2)
I believe the answer is:
blow up
'go off' is the definition.
'riding after shock' is the wordplay.
'riding' becomes 'up' (I've seen this in other clues**).
'after' means one lot of letters go next to another.
'shock' becomes 'blow' ('blow' can be a synonym of 'shock').
'up' after 'blow' is 'BLOW-UP'.
(Other definitions for blow up that I've seen before include "Burst" , "Antonioni film - magnify image" , "Destroy with explosion" , "Enlarge - explode" , "high wind?" .)