In an illicit way, i.e. not permitted by the law or regulations(adverb)
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Use "clandestinely" in a sentence
"And usually the source of support for this guerrilla movement comes clandestinely from a source outside the country under attack."
"With the revelation that NATO didn’t know either, this looks like a directive which may have come clandestinely from the Foreign Affairs or Defense Ministry, but I am growing increasingly skeptical that Stephen Harper did not know."
"But Mr. Mohsen, an American citizen, also loved the idea of clandestinely selling weapons to U.S. allies abroad."