See ark's middle in river decorated with flowers (6)
I believe the answer is:
floral
'decorated with flowers' is the definition.
(I've seen this before)
'see ark's middle in river' is the wordplay.
'see' becomes 'lo' (archaic exclamation meaning 'look!').
'middle' indicates the central letters.
'in' indicates putting letters inside.
'river' becomes 'fal' (Cornish river).
The middle of 'ark' is 'r'.
'lo'+'r'='lor'
'lor' put into 'fal' is 'FLORAL'.
(Other definitions for floral that I've seen before include "Relating to bloomers" , "bouquet" , "sort of arrangement" , "Place for seaside holiday" , "of a Roman goddess" .)