Roman magistrate dismissing a French race (5)
I believe the answer is:
tribe
'race' is the definition.
(I've seen this before)
'roman magistrate dismissing a french' is the wordplay.
'roman magistrate' becomes 'tribune'.
'dismissing' suggests deleting specific letters.
'a french' becomes 'un' ('a' in French).
'tribune' with 'un' taken away is 'TRIBE'.
(Other definitions for tribe that I've seen before include "Social division" , "social set" , "Group of people with a common culture" , "Group of people; big family" , "Racial group living under chief" .)