Model changes diet to complete assignment (2,3,8)
I believe the answer is:
go the distance
'complete assignment' is the definition.
(to go the distance is to complete a task)
'model changes diet to' is the wordplay.
'model' indicates anagramming the letters (letters must be modelled into a new form ).
'changes'+'diet'+'to'='changesdietto'
'changesdietto' with letters rearranged gives 'GO THE DISTANCE'.
(Other definitions for go the distance that I've seen before include "Stay to finish" , "Complete job" , "finish off assignment" , "Complete a boxing match without being knocked out" , "Stay the course" .)