The mantle of sleep is (7)
I believe the answer is:
pelisse
'the mantle' is the definition.
(pelisse is a kind of mantle)
'of sleep is' is the wordplay.
'of' indicates an anagram (the answer comes from or 'of' the letters).
'sleep'+'is'='sleepis'
'sleepis' with letters rearranged gives 'PELISSE'.
(Other definitions for pelisse that I've seen before include "Sleeveless cape" , "coat" , "In a military cloak" , "one worn by lady" , "Long cloak" .)