junior partner
noun[C]
ukusamemberof agroupofpeople,companies,countries, etc. that isinvolvedincontrollingsomething but that has lessinfluenceorexperiencethanothersin thegroup:
Severalforeigncompanieswereinvitedtostayon asjuniorpartnersin thepetroleumfirm.
Thegovernmentcollapsedafter thejuniorpartnerin thecoalitionpulledout.
Compare
senior partner