A bottom one couldn't possibly get beneath? (5)
I believe the answer is:
nadir
'a bottom' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
(Other definitions for nadir that I've seen before include "Drain into the lowest point (5)" , "Lower one cannot get!" , "Low spot" , "Rock bottom" , "Dinar (anag.)" .)