Siemens destroyed long-standing rival (7)
I believe the answer is:
nemesis
'long-standing rival' is the definition.
(a nemesis is a rival or enemy)
'siemens destroyed' is the wordplay.
'destroyed' indicates anagramming the letters.
'siemens' with letters rearranged gives 'NEMESIS'.
(Other definitions for nemesis that I've seen before include "causes downfall" , "E'en miss (anag.)" , "rival who can't be beaten" , "Retribution personified" , "(Fate's) retribution" .)