Complex sentence: Unusually for contemporary clubs, it had a publishing arm, which was instrumental in publishing Cobden's collected speeches in 1870, under the co-editorship of John Bright, one of the club's early patrons.
Split into simpler sentences: Unusually for contemporary clubs, it had a publishing arm. The publishing arm was instrumental in publishing Cobden's collected speeches in 1870, under the co-editorship of John Bright, one of the club's early patrons.