Unknown person going round lake, a killer perhaps (9)
I believe the answer is:
strangler
'a killer perhaps' is the definition.
I can't judge whether this definition defines the answer.
'unknown person going round lake' is the wordplay.
'unknown person' becomes 'stranger' (I've seen this before).
'going round' is an insertion indicator (some letters go around others).
'lake' becomes 'L' (geographical abbreviation).
'stranger' placed around 'l' is 'STRANGLER'.
Can you help me to learn more?
(Other definitions for strangler that I've seen before include "Murderer" , "criminal" , "Killer by choking" , "Killer compressing the windpipe" , "he could take your breath away" .)
