Experts attempt to include knight in family tree (8)
I believe the answer is:
ancestry
'family tree' is the definition.
(ancestry is a kind of family tree)
'experts attempt to include knight' is the wordplay.
'experts' becomes 'aces' (ace is a kind of expert).
'attempt' becomes 'try' (synonyms).
'to include' indicates putting letters inside.
'knight' becomes 'N' (chess abbreviation).
'aces'+'try'='acestry'
'acestry' placed around 'n' is 'ANCESTRY'.
'in' is the link.
(Other definitions for ancestry that I've seen before include "Lineage" , "Line of descendants" , "Genealogy" , "Person's ethnic descent" , "Any crest (anag.)" .)