"Hard as nails,” for one (6)
I believe the answer is:
simile
'hard as nails for one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for simile that I've seen before include "Figure of speech involving comparison" , "Figure of speech in which one thing is likened to another" , "Figure of speech making simple comparison" , "device, comparatively speaking" , "The likening of one thing to another" .)