Long narrow cut or depression (6)
I believe the answer is:
groove
I believe this is a double definition.
'long narrow cut' is the first definition.
(I've seen this before)
'depression' is the second definition.
(groove is a kind of depression)
(Other definitions for groove that I've seen before include "Rut - routine" , "Long narrow channel or kind of rut" , "Cut channel - set routine" , "LP track" , "Long shallow indentation" .)