Taken again by shocker heading north (5)
I believe the answer is:
resat
'taken again' is the definition.
(as in taking an exam again)
'shocker heading north' is the wordplay.
'shocker' becomes 'taser' (something which gives electric shocks).
'heading north' shows that the letters should be reversed in order.
'taser' back-to-front is 'RESAT'.
'by' is the link.
(Other definitions for resat that I've seen before include "Took another chance to pass" , "did test again" , "was tested again" , "Took (exam) again" , "went back to the paper" .)