A model poses for assignments (5)
I believe the answer is:
tasks
'for assignments' is the definition.
(I've seen this before)
'a model poses' is the wordplay.
'a model' becomes 't' (Ford Model T car).
'poses' becomes 'asks' (as in posing or asking a question).
't'+'asks'='TASKS'
(Other definitions for tasks that I've seen before include "Missions" , "Assignments" , "Labours" , "'Jobs, undertakings (5)'" , "Chores, duties" .)