Byword (5)
I believe the answer is:
motto
'byword' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for motto that I've seen before include "Phrase in a Christmas cracker" , "Words often found on coat of arms" , "Repeated theme in music" , "It may be proverbial" , "Favourite saying of a group" .)