Bottom of drain layout (5)
I believe the answer is:
nadir
'bottom' is the definition.
(I've seen this in another clue)
'drain layout' is the wordplay.
'layout' indicates an anagram (an alternative layout of the letters).
'drain' anagrammed 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" .)