Easiest assignments β only some needed rests (7)
I believe the answer is:
siestas
'rests' is the definition.
(a siesta is a small sleep)
'easiest assignments only some' is the wordplay.
'only some' indicates a hidden word.
'SIESTAS' is hidden within 'easiest assignments'.
'needed' is the link.
(Other definitions for siestas that I've seen before include "Assets I hide during naps" , "Afternoon naps" .)