Complex sentence: In December 1988, Wałęsa co-founded the Solidarity Citizens' Committee; ostensibly an advisory body but in practice a political party that won the parliamentary elections in June 1989.
Split into simpler sentences: In December 1988, Wałęsa co-founded the Solidarity Citizens' Committee. Though it was ostensibly an advisory body, in practice it was a political party that won the parliamentary elections in June 1989.