Non-literal expression (5)
I believe the answer is:
'non-literal expression' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for idiom that I've seen before include "Form or variety of language" , "Language used by people or group, vernacular" , "Artistic style" , "Word usage peculiar to a specific group" , "Figure of speech" .)