WordNet (r) 3.0 (2006):

work at
    v 1: to exert effort in order to do, make, or perform something;
         "the child worked at the multiplication table until she had
         it down cold" [syn: work at, work on]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "work at":
   be employed, carry on, carry on business, conduct, do, do business,
   employ, engage in, exercise, follow, follow a trade, go in for,
   have a job, keep up, labor, moonlight, practice,
   practice a profession, prosecute, pursue, set up shop,
   specialize in, stay employed, tackle, take on, take to, take up,
   toil, transact business, undertake, use, wage, work, work for