From Longman Dictionary of Contemporary Englishsparring partnerˈsparring ˌpartner noun [countable] 1 DSsomeone you practise boxing with2 ARGUEsomeone you regularly have friendly arguments with
Examples from the Corpus
sparring partner• Doubt acts as a sparring partner both to truth and error.• The most telling exchanges came between Sir Lewis and his long-time sparring partner, shareholder James Middlemass.• Hollins will be keeping his eye on Councilman Keith Beier, his ideological nemesis and verbal sparring partner on the council dais. 