Phrase on a coat of arms (5)
I believe the answer is:
'phrase on a coat of arms' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for motto that I've seen before include "Maxim; slogan" , "Part of coat of arms" , "Catchphrase" , "Words often found on coat of arms" , "Christmas cracker text" .)