Hold-up as echo's smothered in order to be quiet (5)

I believe the answer is:

heist

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'hold-up' is the definition.
(I know that heist is a type of hold-up)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for heist that I've seen before include "Robbery, often at gunpoint" , "Bank robbery" , "bank job in America?" , "Robbery (mainly US)" , "Mugging" .)

I've seen this clue in The Times.

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