Short period of calm (4)
I believe the answer is:
lull
'short period of calm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lull that I've seen before include "A temporary period of quiet, as in storm" , "Calm interval in storm" , "Put to sleep - respite" , "Short period of quiet" , "Cessation of activity (before a storm?)" .)