noun
key customers; major clients
Business term referring to the most important customers or clients, typically in a supplier–buyer relationship.
当社の主要納入先は大手自動車メーカーです。
Our key customers are major automobile manufacturers.
主要納入先との契約を見直す必要がある。
We need to review the contracts with our major clients.
Compound of 主要 (main, principal) + 納入 (delivery, supply) + 先 (destination, counterparty). The term is a straightforward business compound.