This writer's set took liberties (7)
I believe the answer is:
'took liberties' is the definition.
(to impose on someone is to take liberties)
'this writer's set' is the wordplay.
'this writer's' becomes 'I'm' (the person writing the crossword clues).
'set' becomes 'posed'.
(Other definitions for imposed that I've seen before include "set as a task" , "Forced to be accepted" , "Thrust oneself upon" , "enforced" .)