Sunlit ground across river, not marred by development (8)
I believe the answer is:
unspoilt
'not marred by development' is the definition.
(I've seen this before)
'sunlit ground across river' is the wordplay.
'ground' indicates an anagram (grind the letters ).
'across' is an insertion indicator.
'river' becomes 'po' (Italian river).
'sunlit' is an anagram of 'unsilt'.
'unsilt' placed around 'po' is 'UNSPOILT'.
(Other definitions for unspoilt that I've seen before include "Not marred by development" , "Preserved" , "Virgin" , "Not harmed/damaged" , "Natural - perfect" .)