Short time at defensive ditch (4)
I believe the answer is:
moat
'defensive ditch' is the definition.
(I've seen this before)
'short time at' is the wordplay.
'short time' becomes 'mo' (as in 'just a mo').
'mo'+'at'='MOAT'
(Other definitions for moat that I've seen before include "Last ditch defence?" , "Trench around fortifications" , "Water-filled ditch around stronghold" , "defensive construction" , "Water around castle" .)