Rested for one short day (3)
I believe the answer is:
sat
This clue is a double definition.
'rested' is the first definition.
(to sit is to rest)
'one short day' is the second definition.
(abbreviation for Saturday; 'short' indicates a short form)
'for' is the link.
(Other definitions for sat that I've seen before include "Took a chair" , "Was in session" , "Took (examination)" , "nearly half a day" , "part of the weekend" .)