Lowest point of drain complex (5)
I believe the answer is:
nadir
'lowest point' is the definition.
'drain complex' is the wordplay.
'complex' indicates an anagram.
'drain' with letters rearranged gives 'NADIR'.
'of' is the link.
(Other definitions for nadir that I've seen before include "Drain to lowest level" , "worst-case scenario?" , "The lowest point of anything" , "extremely low position" , "businessman formerly escaping justice" .)