Period of revolution (4)
I believe the answer is:
turn
This clue is a double definition.
'period' is the first definition.
(turn is a kind of period)
'revolution' is the second definition.
(revolution can mean a turn or rotation)
'of' is the link.
(Other definitions for turn that I've seen before include "Move in a new direction" , "revolutionary act" , "Sour" , "Move around an axis, or change" , "roll" .)