WordNet (r) 3.0 (2006):

tip off
    v 1: give insider information or advise to; "He tipped off the
         police about the terrorist plot" [syn: tip off, tip]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "tip off":
   admonish, admonishment, admonition, advice, advise, alarm, alert,
   alerting, breathe, buzz, catchword, caution, caveat, clue, confide,
   confide to, cry havoc, cue, cue word, deterrent example,
   entrust with information, evidence, example, final notice,
   final warning, give confidential information, give fair warning,
   give notice, give warning, hint, hot lead, intimation,
   issue an ultimatum, key, key word, lead, lesson, let in on,
   let next to, mention privately, monition, moral, notice,
   notification, notify, object lesson, office, passing word, point,
   pointer, put hep, put next to, scent, sound the alarm, spoor,
   steer, suggestion, telltale, threat, threaten, tip, trace,
   ultimatum, utter a caveat, verbum sapienti, vestige, warn,
   warn against, warning, warning piece, whiff, whisper