Bottom of drain layout (5)
I believe the answer is:
'bottom' is the definition.
(I've seen this before)
'drain layout' is the wordplay.
'layout' indicates anagramming the letters (an alternative layout of the letters).
'drain' with letters rearranged gives 'NADIR'.
'of' is the link.
(Other definitions for nadir that I've seen before include "deep despair" , "Low point" , "Drain to lowest level" , "Low-water mark" , "Point of greatest despair" .)