Doctor at watery ditch (4)
I believe the answer is:
moat
'watery ditch' is the definition.
(I've seen this in another clue)
'doctor at' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'mo'+'at'='MOAT'
(Other definitions for moat that I've seen before include "Water defence" , "Ditch round castle" , "Ditch dug to protect a castle" , "Last ditch defence?" , "Trench round a castle" .)