Part of flower trimmed, as going to seed (9)
I believe the answer is:
midstream
'part of flower' is the definition.
(part of a river, ie something which flows)
'trimmed as going to seed' is the wordplay.
'going to seed' indicates an anagram (to go to seed can mean to fall apart).
'trimmed'+'as'='trimmedas'
'trimmedas' with letters rearranged gives 'MIDSTREAM'.
(Other definitions for midstream that I've seen before include "At halfway point in current" , "Furthest from banks" , "halfway across the water" , "centre" , "Don't change horses here" .)