As canaries missing a cuckoo (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'canaries missing a cuckoo' is the wordplay.
'missing' indicates named letters should be taken away.
'cuckoo' is an anagram indicator.
'canaries' with 'a' removed is 'cnaries'.
'cnaries' anagrammed gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Element with many poisonous compounds" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Ratsbane" , "As briefly identified" , "killer" .)