Slit at the front (6)
I believe the answer is:
trench
I believe this clue is a double definition.
'slit' is the first definition.
(thesaurus)
'at the front' is the second definition.
(I've seen this before)
(Other definitions for trench that I've seen before include "A military ditch for soldiers" , "deep channel" , "Deeply cut" , "A deep ditch, as dug for troops" , "A ditch dug by soldiers" .)