Calm; quiet period (4)
I believe the answer is:
lull
I believe this clue is a double definition.
'calm' is the first definition.
(I know that calm can be written as lull)
'quiet period' is the second definition.
(I've seen this before)
(Other definitions for lull that I've seen before include "Put to sleep - respite" , "Pause in activity" , "Rock to sleep" , "Calm; allay (fears)" , "Soothe; quiet interval" .)