Fellow almost entirely overwhelmed by daft vernacular (9)
I believe the answer is:
idiomatic
'vernacular' is the definition.
(I've seen this before)
'fellow almost entirely overwhelmed by daft' is the wordplay.
'fellow' becomes 'man' (fellow can mean a man).
'almost entirely' means to remove the last letter (most but not all of the word).
'overwhelmed by' is an insertion indicator (inserted letters get absorbed or overwhelmed).
'daft' becomes 'idiotic' (I've seen this in another clue).
'man' with its final letter taken off is 'ma'.
'ma' put within 'idiotic' is 'IDIOMATIC'.
(Other definitions for idiomatic that I've seen before include "Using characteristic speech" , "Using expressions natural to a native speaker" , "Colloquial" , "manner of speaking" , "Using a particular mode of expression" .)