"Keeping mum" is silly vernacular (9)
I believe the answer is:
idiomatic
'vernacular' is the definition.
(I've seen this before)
'keeping mum is silly' is the wordplay.
'keeping' means one lot of letters goes inside another (some letters kept inside).
'mum' becomes 'ma' (both can mean mother).
'is silly' becomes 'idiotic' (I've seen this before).
'ma' placed into 'idiotic' is 'IDIOMATIC'.
(Other definitions for idiomatic that I've seen before include "vernacular" , "spoken like a native" , "Expressed in distinctive or metaphorical way" , "Using a particular mode of expression" , "Colloquial" .)