Director's left with instructor about to quit (5)
I believe the answer is:
loach
'director's' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for loach that I've seen before include "Fish with barbels around mouth" , "River fish - film director" , "Fish - British film director" , "Film-maker" , "Freshwater fish; rhymes with freshwater fish" .)