Crossword Genius

Screw cap of originality improved lager cans (6)

Ross

I believe the answer is:

gaoler

I'm a little stuck... Click here to teach me more about this clue!

'screw' is the definition.
(I know that screw can be written as gaoler)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in The Sunday Times and The Times.
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