Saw flash catching too much (5)
I believe the answer is:
motto
'saw' is the definition.
(saw can mean a proverb or saying)
'flash catching too much' is the wordplay.
'flash' becomes 'mo' (both mean a very short period of time).
'catching' indicates putting letters inside.
'too much' becomes 'OTT' (short for over the top).
'mo' going around 'ott' is 'MOTTO'.
(Other definitions for motto that I've seen before include "Repeated theme in music" , "Catchcry, slogan" , "Watchword" , "Phrase to express character" , "Christmas cracker text" .)