Saw commercial taking a long time (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'commercial taking a long time' is the wordplay.
'commercial' becomes 'ad'.
'taking' means one lot of letters go next to another.
'a long time' becomes 'age' ('age' can be a synonym of 'long time'**).
'ad'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Proverb expressing a general truth" , "Well-remembered proverb or saying" , "Short statement expressing a general truth" , "say it again and again!" , "Wise old saying" .)