Crossword Genius

Unique old revolutionary loses the head with posh guy needing no introduction (6)

Ross

I believe the answer is:

oneoff

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'unique' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for oneoff that I've seen before include "Single" , "not repeated" , "Unique" .)

I've seen this clue in The Irish Times.
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