Atom bombs? Ring for security (4)
I believe the answer is:
moat
'ring for security' is the definition.
(ring of water around a castle)
'atom bombs?' is the wordplay.
'bombs?' indicates an anagram (I've seen 'bombed' mean this).
'atom' is an anagram of 'MOAT'.
(Other definitions for moat that I've seen before include "Ditch used as a defence" , "Defensive water" , "Water-filled defensive trench" , "Castle's ditch" , "Water around castle" .)