On return, did exam second time? That could be stunning (5)

I believe the answer is:
taser
'second time? that could be stunning' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'on return did exam' is the wordplay.
'on return' shows that the letters should be reversed in order.
'did' becomes 're' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'exam' becomes 'SAT' (standard attainment test, UK exam).
're'+'sat'='resat'
'resat' written backwards gives 'TASER'.
Can you help me to learn more?
(Other definitions for taser that I've seen before include "Weapon firing electrified darts; stare (anag.)" , "PC kit" , "police weapon introduced" , "A shocking thing" , "Device used to stun someone temporarily" .)
