US chemist rotated screen, extracting middle substance (8)
I believe the answer is:
druggist
'us chemist' is the definition.
('druggist' is a US term for a pharmacist)
'extracting middle substance' is the wordplay.
'extracting middle' becomes 'drug' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'substance' becomes 'gist' (synonyms).
'drug'+'gist'='DRUGGIST'
'rotated screen' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Another definition for druggist that I've seen is " Pharmacist (US)".)