Set phrase (5)
I believe the answer is:
idiom
'set phrase' is the definition.
(synonyms)
This is all the clue.
(Other definitions for idiom that I've seen before include "Group of words in which the meaning may not be obvious" , "Linguistic peculiarity" , "Style of speaking characteristic of a specific group" , "Eg, over the moon" , "Characteristic style of a period" .)