Out of practice, head of firm removed (5)

I believe the answer is:

rusty

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'out of practice' is the definition.
(synonyms)

I don't understand the rest of the clue.

(Other definitions for rusty that I've seen before include "(Of iron) affected by damp" , "Antiquated" , "Oxidised, corroded" , "Affected by corrosion - out of practice" , "Having a reddish-orange oxide coating" .)

I've seen this clue in The Sun.

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