Dirty line of soldiers (6)
I believe the answer is:
defile
I believe this clue is a double definition.
'dirty' is the first definition.
(I've seen this in another clue)
'line of soldiers' is the second definition.
(defile could historically mean a line of soldiers)
(Other definitions for defile that I've seen before include "passage" , "Pollute, make dirty" , "Pollute in ravine?" , "Corrupt - mountain pass" , "Foul - pass" .)