Pure flow turned out to be strong (8)
I believe the answer is:
powerful
'strong' is the definition.
(associated in meaning)
'pure flow turned out' is the wordplay.
'turned out' is an anagram indicator (turn out can mean to organise).
'pure'+'flow'='pureflow'
'pureflow' with letters rearranged gives 'POWERFUL'.
'to be' is the link.
(Other definitions for powerful that I've seen before include "Mighty" , "Having a strong effect" , "with lots of horses" , "Having great strength or influence" , "Pure wolf (anag.)" .)