White rag being waved, road closed! That’s concerning (4,6,2)
I believe the answer is:
with regard to
'concerning' is the definition.
(I've seen this in another clue)
'white rag being waved road closed' is the wordplay.
'being waved' indicates anagramming the letters (I've seen 'waves' mean this).
'road' becomes 'rd' (abbreviation).
'closed' becomes 'to' (as in a door closed to).
'white'+'rag'='whiterag'
'whiterag' with letters rearranged gives 'withrega'.
'withrega'+'rd'+'to'='WITH REGARD TO'
'that's' is the link.
(Another definition for with regard to that I've seen is " Touching".)