Nobody attending initially -- director's lowest point (5)
I believe the answer is:
'lowest point' is the definition.
(I've seen this in another clue)
'nobody attending initially director's' is the wordplay.
'initially' indicates taking the first letters.
'director' becomes 'dir'.
The initial letters of 'nobody attending' is 'na'.
(Other definitions for nadir that I've seen before include "worst-case scenario?" , "pits" , "Low spot" , "Drain into the lowest point (5)" , "It's the depths" .)