Man turned up, crossing a defensive ditch (4)
I believe the answer is:
moat
'defensive ditch' is the definition.
(I've seen this before)
'man turned up crossing a' is the wordplay.
'man' becomes 'tom' (**).
'turned up' is a reversal indicator.
'crossing' means one lot of letters goes inside another.
'tom' written backwards gives 'mot'.
'mot' going around 'a' is 'MOAT'.
(Other definitions for moat that I've seen before include "Castle's ditch" , "Water-filled defensive trench" , "Ditch dug to protect a castle" , "Water round a castle" , "Defensive water-filled ditch" .)