Distance down (5)
I believe the answer is:
depth
'distance down' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for depth that I've seen before include "Intensity; profundity" , "Distance from top down as in water" , "Distance from the surface to the bottom" , "Deepness" , "sometimes leading to charge for sailors at war?" .)