Sleep you won't have when dog tired? (6)
I believe the answer is:
catnap
'sleep' is the definition.
(catnapping is a kind of sleeping)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for catnap that I've seen before include "Drop off" , "Kip" , "Brief doze" , "For a rest" , "(Tom's) snooze?" .)