A clan migrating by water (5)
I believe the answer is:
canal
'water' is the definition.
(canal is a kind of water)
'a clan migrating' is the wordplay.
'migrating' is an anagram indicator (I've seen 'migrate' mean this).
'a'+'clan'='aclan'
'aclan' is an anagram of 'CANAL'.
'by' acts as a link.
(Other definitions for canal that I've seen before include "eg Grand Union" , "Manmade flower" , "Man-made strip of water" , "Aqueduct" , "Artificial waterway for barges" .)