Flash a smile (4)
I believe the answer is:
beam
I believe this clue is a double definition.
'flash' is the first definition.
(thesaurus)
'a smile' is the second definition.
(to beam is to smile)
(Other definitions for beam that I've seen before include "Smile radiantly" , "A supportive" , "Ray - side" , "Smile; piece of timber" , "Ray; structural timber" .)