Returning, did test again: shocker! (5)
I believe the answer is:
taser
'shocker' is the definition.
(something which gives electric shocks)
'returning did test again' is the wordplay.
'returning' shows that the letters should be reversed in order.
'did test again' becomes 'resat' (resat an exam).
'resat' back-to-front is 'TASER'.
(Other definitions for taser that I've seen before include "PC kit" , "Weapon firing electrified darts; stare (anag.)" , "Electroshock weapon" , "A shocking thing" , "police weapon introduced" .)