tip off

Short definition from Wiktionary

Verb

1. 1
   To alert or inform (someone), especially confidentially.
2. 2
   To put the ball in play by throwing it up between two opponents.
3. 3
   To put the ball in play by throwing it up between two opponents: To begin.
4. 4
   To pour out (liquor); to tip the vessel and drink until it is empty.
5. 5
   To use a particular brush technique for producing a flawless surface on a wet coat of paint.

Noun

1. 1
   An obvious clue or indication.
2. 2
   A report of suspicious behaviour, especially to an authority.