Coulrophobia

Coulrophobia is the irrational fear of clowns. Luke Smith noted this psychological disorder by its proper name when a clown began appearing to the students at his school. Sarah Jane Smith was a known sufferer from an early age. She had been introduced to the phobia by a toy clown that used to hang near her bed. Smith claimed that its unnatural gaze used to terrify her at night. As an adult, Smith discovered that the toy had, in fact, been manipulated by the Pied Piper to instill this fear in her. (SJA: The Day of the Clown) Ace was also scared of Clowns as she found out on Segonax (DW: The Greatest Show in the Galaxy).

Johnny Depp was also scared of clowns as Luke Smith read in Heat magizine (DW: The Day of the Clown)

Behind the scenes
David Tennant has admitted to suffering from the phobia, citing a sequence in The Deadly Assassin as a moment when he was "proper scared" by Doctor Who as a young viewer.