Every second, a flier's coming in behind schedule (9)
I believe the answer is:
alternate
'every second' is the definition.
('alternate' can be a synonym of 'every second')
'a flier's coming in behind schedule' is the wordplay.
'flier' becomes 'tern' (tern is a kind of birdanimal**).
'coming in' is an insertion indicator.
'behind schedule' becomes 'late' (I've seen this before).
'tern' inserted into 'late' is 'lternate'.
'a'+'lternate'='ALTERNATE'
(Other definitions for alternate that I've seen before include "Go in turns" , "Change to first the one, then the other" , "Substitute" , "Take turns" , "Change from one thing to another by turns" .)