Reason for being milled (6)
I believe the answer is:
ground
This clue is a double definition.
'reason' is the first definition.
(I know that reason can be written as ground)
'milled' is the second definition.
(thesaurus)
'for being' is the link.
(Other definitions for ground that I've seen before include "Arena" , "turned to dust" , "Stadium; crushed" , "foundation course?" , "Bring down" .)