Grin about setter's figure of speech (6)
I believe the answer is:
simile
'figure of speech' is the definition.
(simile is a kind of figure of speech)
'grin about setter's' is the wordplay.
'grin' becomes 'smile' (synonyms).
'about' means one lot of letters goes inside another.
'setter' becomes 'i' (the person that set the crossword).
'smile' going around 'i' is 'SIMILE'.
(Other definitions for simile that I've seen before include "device, comparatively speaking" , "Lies I'm finding in figure of speech" , "Most common figure of speech, one of comparison" , "Basic figure of speech, makes comparison" , "Figure of speech comparing one thing to another" .)