Is about time to rest? (3)
I believe the answer is:
sit
'to rest?' is the definition.
(resting is a kind of sitting)
'is about time' is the wordplay.
'about' says the letters should be written in reverse.
'time' becomes 't'.
'is' backwards is 'si'.
'si'+'t'='SIT'
(Other definitions for sit that I've seen before include "Brood" , "Take weight off feet" , "Take (an examination)" , "Pose for painter" , "as magistrates do" .)