Observe, become aware of (6)
I believe the answer is:
notice
I believe this clue is a double definition.
'observe' is the first definition.
(I know that observe can be written as notice)
'become aware of' is the second definition.
(I've seen this before)
(Other definitions for notice that I've seen before include "Announcement (of the sack?)" , "Advance warning of an event" , "Sign posted in a public place" , "See; advert" , "'Take heed of, perceive (6)'" .)