Start to float down river? (6)
I believe the answer is:
flower
'river?' is the definition.
(a river flows)
'start to float down' is the wordplay.
'start to' indicates taking the first letters.
'down' becomes 'lower' ('lower' can be a synonym of 'down'**).
The first letter of 'float' is 'f'.
'f'+'lower'='FLOWER'
(Other definitions for flower that I've seen before include "rose, perhaps" , "One found in bed perhaps" , "Bloom" , "Plant cultivated for blossoms" , "River" .)