Being stupid I omit to include a phrase (5)
I believe the answer is:
idiom
'a phrase' is the definition.
(an idiom is a phrase or saying)
'being stupid i omit to include' is the wordplay.
'to include' says the answer is hidden in the clue.
'IDIOM' can be found hidden inside 'being stupid i omit'.
(Other definitions for idiom that I've seen before include "Saying specific to a certain language" , "Word usage peculiar to a specific group" , "Form or variety of language" , "Phrase with a meaning that is not literal" , "Local style of expression" .)