Eagle in whirling descent (7)
I believe the answer is:
lineage
'descent' is the definition.
(I know that descent can be written as lineage)
'eagle in whirling' is the wordplay.
'whirling' indicates an anagram.
'eagle'+'in'='eaglein'
'eaglein' with letters rearranged gives 'LINEAGE'.
(Other definitions for lineage that I've seen before include "Ancestry, sequence of descendants" , "Descent from ancestors" , "Descent (from an ancestor)" , "Genealogy" , "The strain" .)