It separates many in a deep sleep (5)
I believe the answer is:
comma
'it separates' is the definition.
The definition suggests a singular noun which matches the answer.
'many in a deep sleep' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'in' means one lot of letters goes inside another.
'a deep sleep' becomes 'coma' (I've seen this before).
'm' going within 'coma' is 'COMMA'.
Can you help me to learn more?
(Other definitions for comma that I've seen before include "I signal a break" , "Short break" , "Punctuation mark indicating slight pause" , "Punctuation mark - butterfly" , "Butterfly with orange and brown wings" .)