It's a long time since he brought you any sleep! (7)
I believe the answer is:
sandman
'long time since he brought you any sleep' is the definition.
The definition suggests a singular noun which matches the answer. I also know that 'sleep' relates to this answer.
'it's a' is the wordplay.
'it's' becomes 'sandm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a' becomes 'an'.
'sandm'+'an'='SANDMAN'
Can you help me to learn more?
(Other definitions for sandman that I've seen before include "Imaginary person who causes children to sleep" , "Sender to sleep of children" , "sleep-inducing agent" , "Fairy who puts children to sleep" , "Children's bedtime fairy" .)