The phrase "only job" refers to the sole or single occupation or responsibility that someone has. It means that the specified job is the only one they have or the most important one. Full definition
Looking for a job is not only a job in and of itself, it's expensive. (distinctiveweb.com)
I received some advice from someone whose only job in life is to take money from «greedy» landlords and give it to «deserving» and «abused» tenants. (biggerpockets.com)
We believe in the transformative power of the everyday dog or cat — «ordinary» companion pets whose only job in life is to love without limits. (petsforpatriots.org)