veteran of the company; old-timer of the firm
Refers to a long-standing, respected senior figure in a company, often with a sense of authority or deep experience. The word 主 (ぬし) here carries a nuance of 'master' or 'owner' in a figurative sense.
As a veteran of the company, he is admired by the younger employees.
Compound of 会社 (company) and 主 (master, owner). The reading ぬし for 主 is a native Japanese word meaning 'master' or 'owner', here used figuratively for a senior figure.