Quick! Tell me what to say! (6)

I believe the answer is:

prompt

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'quick' is the definition.
(I know that quick can be written as prompt)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for prompt that I've seen before include "Give cue to forgetful actor" , "Whisper line to forgetful actor" , "Punctual or timely" , "Immediate - supply forgotten words" , "Remind (actor) of lines" .)

I've seen this clue in the Evening Standard.

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