A fresh flower is opening (8)
I believe the answer is:
aperture
'opening' is the definition.
(aperture is a kind of opening)
'a fresh flower' is the wordplay.
'fresh' becomes 'pert' (I've seen this before).
'flower' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'a'+'pert'+'ure'='APERTURE'
'is' is the link.
(Other definitions for aperture that I've seen before include "Hole, gap" , "Opening, gap" , "Opening, mostly small" , "Crack" , "Narrow opening" .)