Alternate reel (7)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'alternate' is the first definition.
'reel' is the second definition.
('stagger' can be a synonym of 'reel')
(Other definitions for stagger that I've seen before include "Reel about; shock", "Reel; arrange so as to be out of line", "Walk as if drunk", "Walk as if about to fall", "Reel; amaze", "Reel - astound", "Walk unsteadily, from weakness or drink", "Walk as though in danger of falling", "Reel, walk unsteadily" and "Teeter".)