Lowest point of a drain? (5)
I believe the answer is:
nadir
'lowest point' is the definition.
(I've seen this before)
'of a drain?' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'drain' anagrammed gives 'NADIR'.
(Other definitions for nadir that I've seen before include "You can't sink lower!" , "Very bottom, farthest from zenith" , "Drain to lowest level" , "Lower one cannot get!" , "Drain into the lowest point (5)" .)