Saw commercial over time (5)
I believe the answer is:
adage
'saw' is the definition.
(saw can mean a proverb or saying)
'commercial over time' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'over' says to put letters next to each other.
'time' becomes 'age' (an age is a period of time).
'ad'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Short statement expressing a general truth" , "Wise old saying" , "say it again and again!" , "Dictum" , "Well-remembered proverb or saying" .)