What cab, but not bus, has for sound of horn? (5)

I believe the answer is:

blast

'sound of horn?' is the definition.

I cannot understand how the remainder of the clue works.

(Other definitions for blast that I've seen before include "Sudden strong gust of wind" , "Damn - explosion" , "Severe reprimand; strong gust" , "Loud horn sound" , "Mild expletive - sharp explosion" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app