Siemens' misguided and 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 anagramming the letters (letters guided into the wrong places).
'siemens' is an anagram of 'NEMESIS'.
'and' is the link.
(Other definitions for nemesis that I've seen before include "An agent of vengeance and retribution" , "Agent of (someone's) downfall" , "enemy" , "Unbeatable rival" , "curse" .)