Rested, then returned to an incomplete task (3)
I believe the answer is:
sat
'rested' is the definition.
(I've seen this before)
'returned to an incomplete task' is the wordplay.
'returned' says the letters should be written in reverse.
'to an incomplete' means to remove the last letter.
'task' with its final letter taken away is 'tas'.
'tas' in reverse letter order is 'SAT'.
'then' acts as a link.
(Other definitions for sat that I've seen before include "Was couch potato" , "Perched" , "Took (an exam)" , "Posed (for an artist)" , "Relaxed" .)