Sleep is something easy to do! (4)
I believe the answer is:
doss
This clue is a double definition.
'sleep' is the first definition.
(I've seen this before)
'something easy to do' is the second definition.
(if something is a doss it is extremely easy)
'is' is the link.
(Other definitions for doss that I've seen before include "go to bed" , "easily accomplished" , "going off to sleep" , "Sleep somewhere convenient" , "Sleep like a tramp" .)