Again and again notice and yet again scan (6)
I believe the answer is:
reread
'again scan' is the definition.
('read' can be a synonym for 'scan' so perhaps 'reread' is OK for 'again scan')
'again and again notice' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'and again' indicates the doubling of some letters.
'notice' becomes 'ad' (both can mean an advertisement).
're' doubled is 'rere'.
'rere'+'ad'='REREAD'
'and yet' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for reread that I've seen before include "look again at manifesto?" , "Study text again" , "Perused again" , "Peruse or scan again; it's dearer" , "Give script second seeing to" .)