Deepness (5)
I believe the answer is:
depth
'deepness' is the definition.
(I know that deepness can be written as depth)
This is the entire clue.
(Other definitions for depth that I've seen before include "Distance downwards" , "Distance from top down as in water" , "Military life; vulgarly artificial" , "Profoundness" , "Distance from the surface to the bottom" .)