Group related by blood or marriage (5)
'group related by blood or marriage' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tribe that I've seen before include "Native group" , "A set or class of people, may be primitive" , "Extended family group" , "Cultural society division" , "Group of related people" .)
(Other definitions for clan that I've seen before include "One from Benghazi" , "Scottish tribe" , "Extended family as in Scotland" , "Kinship group" , "Scots family grouping" .)