Siemens engineer is an unbeatable opponent (7)
I believe the answer is:
nemesis
'an unbeatable opponent' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'siemens engineer' is the wordplay.
'engineer' indicates an anagram.
'siemens' anagrammed gives 'NEMESIS'.
'is' is the link.
Can you help me to learn more?
(Other definitions for nemesis that I've seen before include "E'en miss (anag.)" , "Bane" , "Fate" , "Figure of retribution" , "Greek god of vengeance" .)