Siemens’ misguided, long-standing rival (7)

I believe the answer is:

nemesis

'long-standing rival' is the definition.
(a nemesis is a rival or enemy)

'siemens' misguided' is the wordplay.
'misguided' indicates an anagram (letters guided into the wrong places).
'siemens' with letters rearranged gives 'NEMESIS'.

(Other definitions for nemesis that I've seen before include "Agent of (someone's) downfall" , "Divinity" , "Unbeatable rival" , "Retribution personified" , "Avenging deity" .)

I've seen this clue in The Times.

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