Sly as a fox, e.g. (6)
I believe the answer is:
simile
(Other definitions for simile that I've seen before include "The likening of one thing to another" , "Figure of speech comparing one thing to another" , "Figure of speech, eg, as cool as a cucumber" , "Basic figure of speech, makes comparison" , "Figure of speech in which one thing is likened to another" .)