Every second, a bird comes in dead (9)
I believe the answer is:
'every second' is the definition.
'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'.
(Other definitions for alternate that I've seen before include "Go in turns" , "Substitute" , "Occurring in turn repeatedly" , "Act reciprocally" , "Interchange" .)