Strangler's device police informer carries round (5)
I believe the answer is:
noose
'strangler's device' is the definition.
(a strangler might use a noose)
'police informer carries round' is the wordplay.
'police informer' becomes 'nose' (slang term).
'carries' is an insertion indicator (inserted letters carried inside).
'round' becomes 'o' (round can mean a round object).
'nose' placed around 'o' is 'NOOSE'.
(Other definitions for noose that I've seen before include "Cord snare" , "Loop formed by means of a slip knot" , "Hangman's running loop" , "Snare made of rope" , "Loop for catching animals" .)