First appearance of cuckoo in a nest - bird ousting one as of old (9)
I believe the answer is:
ancestral
'ousting one as of old' is the definition.
I can't judge whether this defines the answer.
'first appearance of cuckoo in a nest bird' is the wordplay.
'first appearance of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'bird' becomes 'ral' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letter of 'cuckoo' is 'c'.
'a'+'nest'='anest'
'c' going into 'anest' is 'ancest'.
'ancest'+'ral'='ANCESTRAL'
Can you help me to learn more?
(Other definitions for ancestral that I've seen before include "Lancaster (anag)" , "by old relations" , "Like many an old family home" , "Coming down the line" , "Relating to one's forebears" .)