Lowest point of drain layout (5)
I believe the answer is:
nadir
'lowest point' 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' anagrammed gives 'NADIR'.
'of' acts as a link.
(Other definitions for nadir that I've seen before include "Low-water mark" , "Point of greatest despair" , "Lowest point, or time of deepest depression." , "Lower one cannot get!" , "Low point" .)