Observing and understanding (6)
I believe the answer is:
seeing
This clue is a double definition.
'observing' is the first definition.
(I've seen this before)
'understanding' is the second definition.
('see' can be a synonym of 'understand')
'and' acts as a link.
(Other definitions for seeing that I've seen before include "Perceiving by eye" , "Watching" , "... is believing, it's said" , "Meeting" , "since" .)