‘Midi’ coat I style, in the vernacular (9)
I believe the answer is:
idiomatic
'the vernacular' is the definition.
(I've seen this before)
'midi coat i style' is the wordplay.
'style' indicates an anagram (style can mean to shape or design).
'midi'+'coat'+'i'='midicoati'
'midicoati' with letters rearranged gives 'IDIOMATIC'.
'in' acts as a link.
(Other definitions for idiomatic that I've seen before include "Certain phrases may be" , "using dialect" , "Using a particular mode of expression" , "manner of speaking" , "vernacular" .)