A clan flourished by some water (5)
I believe the answer is:
canal
'some water' is the definition.
(canal is a kind of water)
'a clan flourished' is the wordplay.
'flourished' indicates an anagram (I've seen this in other clues).
'a'+'clan'='aclan'
'aclan' anagrammed gives 'CANAL'.
'by' acts as a link.
(Other definitions for canal that I've seen before include "Inland waterway" , "Man-made strip of water" , "Duct" , "Aqueduct" , "Passage" .)