Drain designed to reach the lowest point (5)
I believe the answer is:
'the lowest point' is the definition.
(I've seen this before)
'drain designed' is the wordplay.
'designed' is an anagram indicator (designed with the letters).
'drain' is an anagram of 'NADIR'.
'to reach' acts as a link.
(Other definitions for nadir that I've seen before include "Drain (anag)" , "It's the depths" , "Drain into the lowest point (5)" , "most unsuccessful point" , "Low spot" .)