Ring through snout is topping device (5)
I believe the answer is:
noose
'topping device' is the definition.
(to 'top oneself' is to kill oneself)
'ring through snout' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'snout' becomes 'nose' (informal term for nose).
'o' inserted into 'nose' is 'NOOSE'.
'is' acts as a link.
(Other definitions for noose that I've seen before include "Loop with running knot" , "old execution method" , "Loop in a lariat" , "trap" , "Cord snare" .)