tip-off

noun
  1. 1

    An obvious clue or indication.

    โ€œThe broken window and overturned plant pots were a tip-off that something was wrong.โ€

  2. 2

    A report of suspicious behaviour, especially to an authority.

    โ€œThe police received a tip-off about a recent bank robbery.โ€

Source: https://en.wiktionary.org/wiki/tip-off