Characteristic style of expression (5)
I believe the answer is:
idiom
'characteristic style of expression' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for idiom that I've seen before include "Form or variety of language" , "Phrase with a meaning that is not literal" , "Word usage peculiar to a specific group" , "Colloquialism" , "Phrase such as over the moon" .)