Every second, a bird comes in dead (9)
I believe the answer is:
alternate
'every second' is the definition.
(synonyms)
'a bird comes in dead' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'comes in' indicates putting letters inside.
'dead' becomes 'late' (similar in meaning).
'tern' inserted inside 'late' is 'lternate'.
'a'+'lternate'='ALTERNATE'
(Other definitions for alternate that I've seen before include "Go in turns" , "Substitute" , "Occurring in turn repeatedly" , "Act reciprocally" , "Interchange" .)