Occasionally calm second rounds (4)
I believe the answer is:
ammo
'rounds' is the definition.
(rounds of ammunition)
'occasionally calm second' is the wordplay.
'occasionally' means one should take alternating letters.
'second' becomes 'mo' (both are short periods of time).
The alternating letters of 'calm' are 'am'.
'am'+'mo'='AMMO'
(Other definitions for ammo that I've seen before include "Slugs perhaps" , "Bullets, shells (colloq.)" , "essential supplies for soldiers" , "Bullets (sl.)" , "Explosive devices" .)