Fantastic eagle in descent (7)
I believe the answer is:
lineage
'descent' is the definition.
('lineage' can be a synonym of 'descent')
'fantastic eagle in' is the wordplay.
'fantastic' indicates anagramming the letters.
'eagle'+'in'='eaglein'
'eaglein' anagrammed gives 'LINEAGE'.
(Other definitions for lineage that I've seen before include "Genealogy" , "Direct descent" , "Stock" , "Ancestral line as seen in eagle" , "Pedigree, family tree" .)