One is conscious of going without it (5)
I believe the answer is:
sleep
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sleep that I've seen before include "Be dormant" , "Slumber" , "''... that knits up the ravell'd sleeve of care'' (Macbeth)" , "Drop off" , "Snooze" .)