Model boats moved around river in vital flow (11)
I believe the answer is:
bloodstream
'vital flow' is the definition.
(the flow of blood is vital for life)
'model boats moved around river' is the wordplay.
'moved' is an anagram indicator.
'around' indicates putting letters inside.
'river' becomes 'r'.
'model'+'boats'='modelboats'
'modelboats' is an anagram of 'bloodsteam'.
'bloodsteam' placed around 'r' is 'BLOODSTREAM'.
'in' acts as a link.
(Other definitions for bloodstream that I've seen before include "vital passage" , "It circulates in vessels" , "something essential to human life" , "current in vessels" , "Vital channel" .)