Sleeps over cover (4)
I believe the answer is:
span
'cover' is the definition.
(I've seen this before)
'sleeps over' is the wordplay.
'sleeps' becomes 'naps' ('nap' can be a synonym of 'sleep').
'over' says the letters should be written in reverse.
'naps' backwards is 'SPAN'.
(Other definitions for span that I've seen before include "Full extent end to end" , "Extend across, bridge" , "Extent of bridge or life" , "Extent, space for odd pans" , "Team of two matched oxen" .)