Said I omitted part of the colloquialism (5)
I believe the answer is:
idiom
'the colloquialism' is the definition.
(I've seen this before)
'said i omitted part' is the wordplay.
'part' says the answer is hidden in the clue.
'IDIOM' is hidden within 'said i omitted'.
'of' is the link.
(Other definitions for idiom that I've seen before include "Expression that's characteristic of particular group" , "Saying specific to a certain language" , "Style of speaking characteristic of a specific group" , "Form of linguistic expression" , "Characteristic style of a period" .)