Unsettling observer briefly admits "I'm not sure" (4)
I believe the answer is:
eery
'unsettling' is the definition.
(alternative spelling of 'eerie')
'observer briefly admits i'm not sure' is the wordplay.
'observer' becomes 'eye' (organ which sees or observes).
'briefly' means to remove the last letter.
'admits' indicates putting letters inside.
'i'm not sure' becomes 'er' (expression of hesitation).
'eye' with its final letter taken off is 'ey'.
'ey' enclosing 'er' is 'EERY'.
(Other definitions for eery that I've seen before include "Weirdly frightening" , "Weird" , "Strangely frightening" , "Strange" , "Spooky" .)