Family tree reveals this sudden visit (7)
I believe the answer is:
descent
'family tree' is the definition.
(descent is a kind of family tree)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for descent that I've seen before include "Pedigree" , "that concerns genealogist?" , "Derivation from an ancestor - going down" , "Decline" , "Comedown (of ancestry?)" .)