Some among relatives of mixed blood (7)
I believe the answer is:
mongrel
'of mixed blood' is the definition.
(I've seen this before)
'some among relatives' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'MONGREL' can be found hidden inside 'among relatives'.
(Other definitions for mongrel that I've seen before include "of unknown variety" , "bastard" , "mixed stock" , "Cur" , "Non-pedigree" .)