[Extremely stormy night here: movement's not natural] (9)
I believe the answer is:
synthetic
'not natural' is the definition.
(something synthesised is not natural)
'extremely stormy night here movement's' is the wordplay.
I cannot really understand how this works, but
'movement' could be 'tic' (I have seen 'Nervous movement' mean 'tic' so perhaps 'movement' could also mean 'tic') and 'tic' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for synthetic that I've seen before include "Mock" , "Artificial, man-made compound" , "Man-made (fibre)" , "substance" , "Not genuine, unnatural" .)