Change by turns every second (9)
I believe the answer is:
alternate
I believe this clue is a double definition.
'change by turns' is the first definition.
(I've seen this before)
'every second' is the second definition.
('alternate' can be a synonym of 'every second')
(Other definitions for alternate that I've seen before include "Go in turns" , "Change from one thing to another by turns" , "Substitute" , "deputy" , "Interchange" .)