Not visible (6)
'not visible' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for unseen that I've seen before include "Unnoticed" , "not exercising pupils?" , "not processed by pupils?" , "Lurking" , "Translation exercise" .)
(Other definitions for imperceptible that I've seen before include "Too slight to be noticed" , "Barely detectable" , "Not discernible by the senses" , "Impossible to feel" , "Unnoticeable" .)