Not observed (6)
'not observed' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unseen that I've seen before include "Invisable" , "Invisible or not observed" , "Not noticed" , "Translation exercise" , "Hidden from view" .)
(Other definitions for unnoticed that I've seen before include "Unperceived" , "Not registered" , "behind closed doors?" , "without attracting attention" , "Overlooked" .)