See sign (6)
I believe the answer is:
notice
This clue is a double definition.
'see' is the first definition.
(notice can mean to see or realise)
'sign' is the second definition.
(I know that notice is a type of sign)
(Other definitions for notice that I've seen before include "Announcement or warning" , "Advance information" , "Sign posted in a public place" , "Poster; observe" , "'Observe, discern (6)'" .)