the.com/hired gun

means A specialist brought in from outside—for money, not mission—to do a job someone else can't or won't do themselves.

from Straight out of the American West: a gunslinger paid to fight someone else's battle, no personal stake in it beyond the fee. By the mid-1900s the phrase had holstered its literal gun and ridden into boardrooms and newsrooms, describing any mercenary expert—a lawyer, consultant, or ghostwriter—summoned for their skill and sent away when the job's done.