Thing to do (8)
(Other definitions for activity that I've seen before include "Busyness" , "Occupation or exercise" , "Pastime or bustle" , "Something performed for recreation or exercise" , "Recreational pursuit" .)
'thing to do' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for task that I've seen before include "Give mission" , "Chore, job." , "Piece of work to be done" , "Allocate work" , "Assignment" .)