Lay it on with "I am to act as model" (6)
I believe the answer is:
impose
'lay' is the definition.
(thesaurus)
'as model' is the wordplay.
'as' becomes 'im' (I can't explain this - if you can you should believe this answer much more).
'model' becomes 'pose' (a model poses for photographs).
'im'+'pose'='IMPOSE'
'it on with i am to act' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for impose that I've seen before include "Exact or levy, say tax" , "Be a bother" , "institute" , "Force on or take advantage of" , "Put burden or tax on" .)