Longman Dictionary of Contemporary English - mainstay

~ n the mainstay of a) an important part of something that makes it possible for it to work properly or continue to exist  (Agriculture is still the mainstay of the country's economyb)someone who does most of the important work for a group or organization)  (She was the mainstay of the team.) b) someone who does most of the important work for a group or organization  (She was the mainstay of the team.)