Such expressions may be meaningless, academic admitted (9)
I believe the answer is:
idiomatic
'such expressions may be' is the definition.
(as in an idiomatic expression)
'meaningless academic admitted' is the wordplay.
'meaningless' becomes 'idiotic' (both can mean inane or foolish).
'academic' becomes 'ma'.
'admitted' is an insertion indicator.
'idiotic' placed around 'ma' is 'IDIOMATIC'.
(Other definitions for idiomatic that I've seen before include "Using a particular mode of expression" , "spoken like a native" , "using dialect" , "Using expressions natural to a native speaker" , "Using common language" .)