The Collaborative International Dictionary of English v.0.48:

Advocacy \Ad"vo*ca*cy\, n. [OF. advocatie, LL. advocatia. See
   Advocate.]
   The act of pleading for or supporting; work of advocating;
   intercession.
   [1913 Webster]

WordNet (r) 3.0 (2006):

advocacy
    n 1: active support of an idea or cause etc.; especially the act
         of pleading or arguing for something [syn: advocacy,
         protagonism]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "advocacy":
   abetment, admonition, advice, advising, advocating, advocation,
   aegis, auspices, backing, briefing, bruiting, buildup, care,
   caution, caveat, certificate of character, championship, character,
   character reference, charity, consultation, council, counsel,
   countenance, credential, direction, drumbeating, encouragement,
   exhortation, expostulation, favor, flack, fosterage, goodwill,
   guidance, hortation, idea, instruction, interest,
   letter of introduction, monition, opinion, parley, patronage,
   promoting, promotion, proposal, publicization, publicizing,
   recommend, recommendation, reference, remonstrance, seconding,
   sponsorship, suggestion, sympathy, testimonial, thought, tutelage,
   voucher, warning