Deep ditch (6)
I believe the answer is:
trench
'deep ditch' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trench that I've seen before include "It may be slit" , "Ditch dug as fortification" , "A deep ditch, as dug for troops" , "Drain" , "Long, narrow excavation" .)