Grin about one bright as a button say (6)
I believe the answer is:
simile
'bright as a button say' is the definition.
I can't judge whether this defines the answer.
'grin about one' is the wordplay.
'grin' becomes 'smile' (synonyms).
'about' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'smile' going around 'i' is 'SIMILE'.
Can you help me to learn more?
(Other definitions for simile that I've seen before include "Basic figure of speech, makes comparison" , "one being as good as gold" , "Figure of speech involving comparison" , "Figure of speech used in making comparisons" , "Lies I'm finding in figure of speech" .)