Adult with fresh flower opening (8)
I believe the answer is:
aperture
'opening' is the definition.
(aperture is a kind of opening)
'adult with fresh flower' is the wordplay.
'adult' becomes 'a' (abbreviation).
'with' is a charade indicator (letters next to each other).
'fresh' becomes 'pert' (I've seen this in other clues).
'flower' becomes 'ure' (river in Yorkshire).
'a'+'pert'+'ure'='APERTURE'
(Other definitions for aperture that I've seen before include "'Hole, opening (8)'" , "Narrow opening" , "An opening that admits light" , "Opening (on camera, for example)" , "Eye" .)