Broad beam
I believe the answer is:
'broad beam' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for grin that I've seen before include "Ring (anag.)" , "Smile while pulling back the lips" , "Facial expression showing amusement or pleasure" , "Smile - all that could be seen of the vanishing Cheshire Cat" , "11 smile" .)