MOAT

Water-filled defensive trench (4)

Solution has appeared in 2 crosswords (1 across and 1 down):

• Quick crossword No 15,998 — 2021-08-15 23:00:20
  20D: Defensive ditch, usually filled with water (4)
• Quick crossword No 15,239 — 2019-03-12 00:00:25
  20A: Water-filled defensive trench (4)