Quiet period (4)
I believe the answer is:
'quiet period' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lull that I've seen before include "Calm - calm by deception" , "A temporary pause as in storm" , "Period of inactivity" , "Cessation of activity (before a storm?)" , "A temporary period of quiet, as in storm" .)