Phrase (5)
I believe the answer is:
idiom
'phrase' is the definition.
(an idiom is a phrase or saying)
This is the entire clue.
(Other definitions for idiom that I've seen before include "Eg, over the moon" , "Phrase such as over the moon" , "Linguistic peculiarity" , "'Break a leg' or 'miss the boat'" , "Expression that does not make literal sense" .)