Stunner posed again, given rise (5)
I believe the answer is:
taser
'stunner' is the definition.
(weapon which stuns or shocks)
'posed again given rise' is the wordplay.
'posed again' becomes 'resat' ('sat' is an answer for 'posed' so perhaps 'resat' is OK for 'posed again').
'given rise' shows that the letters should be reversed in order (in a down clue, letters go up).
'resat' backwards is 'TASER'.
(Other definitions for taser that I've seen before include "Device used to stun someone temporarily" , "Gun firing electrified barbs" , "Weapon firing electrified darts; stare (anag.)" , "Stunning device" , "PC kit" .)