Lowest point of a drain, possibly (5)
I believe the answer is:
nadir
'lowest point' is the definition.
(I've seen this before)
'drain possibly' is the wordplay.
'possibly' is an anagram indicator.
'drain' anagrammed gives 'NADIR'.
'of a' acts as a link.
(Other definitions for nadir that I've seen before include "It's the depths" , "worst-case scenario?" , "deep despair" , "All-time low" , "The lowest point of anything" .)