Understood to have been a stalker (8)
I believe the answer is:
followed
This might be a double definition.
'understood' is the first definition.
(I've seen this in another clue)
'been a stalker' is the second definition.
'followed' can be an answer for 'been' (following is a kind of being). I am not sure about the 'a stalker' bit.
'to have' is the link.
Can you help me to learn more?
(Other definitions for followed that I've seen before include "Travelled behind or ensued" , "Came along behind" , "Pursued" .)