Film’s original poster writer, the guy one’s flipping about (11)
I believe the answer is:
oppenheimer
'film's original' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'poster writer the guy one's flipping about' is the wordplay.
I cannot really understand how this works, but
'writer' could be 'pen' (something which writes) and 'pen' is found within the answer.
'one's' could be 'im' ('one is' is a formal way of saying 'I am') and 'im' is found within the answer.
'the guy' could be 'he' and 'he' is found within the answer.
The remaining letters 'oper' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for oppenheimer that I've seen before include "Atomic expert" , "Nuclear physicist" .)