Mode of expression (5)
I believe the answer is:
idiom
'mode of expression' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for idiom that I've seen before include "Form or variety of language" , "Word usage peculiar to a specific group" , "Phrase with a meaning that is not literal" , "Characteristic style of a period" , "Linguistic peculiarity" .)