It's turned (by a turnkey?) (5)
I believe the answer is:
screw
'turnkey?' is the definition.
('screw' can be a synonym of 'turnkey')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for screw that I've seen before include "Prison guard, colloquially" , "Old horse" , "Fixer; twist of paper" , "Archimedes' invention" , "Bring pressure to bear" .)