They murder visitors around lake (10)
I believe the answer is:
stranglers
'they murder' is the definition.
'visitors around lake' is the wordplay.
'visitors' becomes 'strangers'.
'around' means one lot of letters goes inside another.
'lake' becomes 'l'.
'strangers' placed around 'l' is 'STRANGLERS'.
(Other definitions for stranglers that I've seen before include "They'll put the squeeze on" , "murderers!" .)