Low-water mark (5)
I believe the answer is:
nadir
'low-water mark' is the definition.
(I know that low-water mark can be written as nadir)
This is all the clue.
(Other definitions for nadir that I've seen before include "businessman formerly escaping justice" , "Point opposite the zenith" , "deep despair" , "Dinar (anag.)" , "Floor of drain in need of repair" .)