Ditch – ___ coat (6)
I believe the answer is:
trench
I believe this clue is a double definition.
'ditch' is the first definition.
(I know that trench is a type of ditch)
'coat' is the second definition.
(trench coat is a kind of coat)
(Other definitions for trench that I've seen before include "A deep ditch, as dug for troops" , "Long, narrow excavation" , "Drain" , "deep channel" , "depression" .)