Saw how old notice is? (5)
I believe the answer is:
adage
'saw' is the definition.
(saw can mean a proverb or saying)
'how old notice is?' is the wordplay.
I cannot really understand how this works, but
'notice' could be 'ad' (both can mean an advertisement) and 'ad' is located in the answer.
'how old' could be 'age' (I've seen this before) and 'age' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for adage that I've seen before include "Slogan" , "say it again and again!" , "Well-remembered proverb or saying" , "Wise old saying" , "Maxim, aphorism" .)