Poser with poem written in 1050 (5)
I believe the answer is:
model
'poser' is the definition.
(e.g. a model for an artist - someone who poses)
'poem written in 1050' is the wordplay.
'poem' becomes 'ode' (ode is a type of poem).
'written in' indicates putting letters inside.
'm050' becomes 'ml' (I can't explain this - if you can you should give a lot more credence to this answer).
'ode' going inside 'ml' is 'MODEL'.
'with' acts as a link.
(Other definitions for model that I've seen before include "(Person functioning as) example" , "One working for painter or couturier" , "Scale representation" , "Fashion - prototype" , "Small-size version" .)