Saw how old Bill is? (5)
I believe the answer is:
adage
'saw' is the definition.
(saw can mean a proverb or saying)
'how old bill is?' is the wordplay.
I cannot quite see how this works, but
'bill' could be 'ad' (advertisement stuck on wall) and 'ad' is found within the answer.
'how old' could be 'age' (I've seen this before) and 'age' is present in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for adage that I've seen before include "Proverb, maxim" , "Proverb expressing a general truth" , "Saw, proverb" , "True proverb or saying" , "Old saying, maxim" .)