Getting the gist as you go off to sleep (5)
I believe the answer is:
drift
'sleep' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for drift that I've seen before include "Large heap (of snow)" , "non-standard deviation?" , "Go with the current" , "Bank (of snow)" , "Implication of what's been said" .)