Flower planted in sediment is ruined (6)
I believe the answer is:
'is ruined' is the definition.
'flower planted in sediment' is the wordplay.
'flower' becomes 'po' (Italian river).
'planted in' is an insertion indicator.
'sediment' becomes 'silt' (I've seen this before).
'po' inserted within 'silt' is 'SPOILT'.
(Other definitions for spoilt that I've seen before include "Deprived" , "Destroyed the value of" , "Overindulged" , "Marred" , "Pampered" .)