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 quite see how this works, but
'notice' could be 'ad' (both can mean an advertisement) and 'ad' is present in the answer.
'how old' could be 'age' (I've seen this before) and 'age' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
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" .)