Half came, one in twelve descended from French colonists (5)
I believe the answer is:
cajun
'descended from french colonists' is the definition.
(Louisiana group originating from French colonists)
'half came one in twelve' is the wordplay.
'half' means to take half the letters.
'one in twelve' becomes 'jun' (abbreviation for June - one of twelve months).
'came' with half the letters taken is 'ca'.
'ca'+'jun'='CAJUN'
(Other definitions for cajun that I've seen before include "One of the 9 [BAYOU] people" , "Kind of spicy food" , "French dialect of Louisiana" , "French-speaking Louisiana native" , "Louisianan of French Canadian descent" .)