Every second a flyer goes in behind schedule (9)
I believe the answer is:
alternate
'every second' is the definition.
('alternate' can be a synonym of 'every second')
'a flyer goes in behind schedule' is the wordplay.
'flyer' becomes 'tern'.
'goes in' indicates putting letters inside.
'behind schedule' becomes 'late' (I've seen this before).
'tern' placed into 'late' is 'lternate'.
'a'+'lternate'='ALTERNATE'
(Other definitions for alternate that I've seen before include "Substitute" , "Change to first the one, then the other" , "go back and forth" , "deputy" , "Act reciprocally" .)