The saying to remember is "When in doubt, Cite."
Citing any source can save you a lot of bother. Not citing a source can lead you into court.
Clip-Art is no different, it is someone else's intellectual material so it is good to cite it.
Cite it as from Word, give the URL if it is a web based clip-art source, if there is no named person as originator then give Clip-Art as the originator.