Cuckoo nest he reveals has no adult yet (12)
I believe the answer is:
nevertheless
'yet' is the definition.
(I know that yet can be written as nevertheless)
'cuckoo nest he reveals has no adult' is the wordplay.
'cuckoo' is an anagram indicator (I've seen this in other clues).
'has no' indicates named letters should be taken away.
'adult' becomes 'a' (abbreviation).
'nest'+'he'+'reveals'='nesthereveals'
'nesthereveals' with 'a' removed is 'nestherevels'.
'nestherevels' with letters rearranged gives 'NEVERTHELESS'.
(Other definitions for nevertheless that I've seen before include "anyhow" , "In spite of everything" , "Still" , "However" , "But always" .)