work at something

(つとめる) Make an effort (at improving your French, at quitting smoking, to be nice to your friend’s douche boyfriend, etc.), more like 頑張る than the other つとめるs.   $$$
(つとめる) Perform a specific task or role which you have been assigned (but not necc. at a job)
(つとめる) Work (used only about a job, and not in reference to a particular project at that job) (emphasis is on the physical work rather than the role).