Fancy crest adopted by one unspecified family (8)
I believe the answer is:
ancestry
'family' is the definition.
(I've seen this before)
'fancy crest adopted by one unspecified' is the wordplay.
'fancy' indicates anagramming the letters.
'adopted by' is an insertion indicator (I've seen this in other clues).
'one unspecified' becomes 'any' (any example of something).
'crest' with letters rearranged gives 'cestr'.
'cestr' put inside 'any' is 'ANCESTRY'.
(Other definitions for ancestry that I've seen before include "Blood line" , "race" , "Genealogy" , "Family tree" , "Line of forebears" .)