Opening cocktail mixer, one becomes more unsteady (7)

I believe the answer is:

shakier

'more unsteady' is the definition.
(shaky can mean unsteady)

'opening cocktail mixer one' is the wordplay.
'opening' indicates putting letters inside (some letters open a gap in others).
'cocktail mixer' becomes 'shaker' (container for shaking cocktails).
'one' becomes 'I' (Roman numeral).
'shaker' enclosing 'i' is 'SHAKIER'.

'becomes' acts as a link.

I've seen this clue in The Times.

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