Grounds covered by area, so natural (6)
I believe the answer is:
reason
'grounds' is the definition.
(I've seen this before)
'covered by area so natural' is the wordplay.
'covered by' indicates a hidden word.
'REASON' is hidden within 'area so natural'.
(Other definitions for reason that I've seen before include "Motivation or justification" , "'Motivation, grounds (6)'" , "Justification needed" , "Cause; think logically" , "Justification (for action)" .)