Lowest or deepest point (5)
I believe the answer is:
nadir
This clue is a double definition.
'lowest' is the first definition.
(I've seen this before)
'deepest point' is the second definition.
(I've seen this before)
(Other definitions for nadir that I've seen before include "lowest of the low" , "All-time low" , "Point of greatest despair" , "Drain to lowest level" , "Point opposite the zenith" .)