Colloquial saying (5)
I believe the answer is:
'colloquial saying' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for idiom that I've seen before include "Dialect, vernacular" , "Phrase with a meaning that is not literal" , "Linguistic peculiarity" , "Characteristic style of expression" , "Group of words in which the meaning may not be obvious" .)