Many old assault threats initially — prevented by this? (4)
I believe the answer is:
moat
'prevented by this?' is the definition.
The definition suggests a singular noun which matches the answer.
'many old assault threats initially' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'initially' suggests taking the first letters.
The initial letters of 'old assault threats' is 'oat'.
'm'+'oat'='MOAT'
(Other definitions for moat that I've seen before include "Ditch dug to protect a castle" , "A deep water-filled ditch around a castle" , "Defensive water-filled ditch" , "Castle's ditch" , "Water-filled defensive trench" .)