An immediate reminder of what to say (6)

I believe the answer is:

prompt

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'an immediate' is the definition.
(I know this)

I cannot understand how the rest of the clue works.

(Other definitions for prompt that I've seen before include "Bring about; actors' help" , "Immediate - supply actor's lines" , "Supply a forgotten line to an actor" , "Jog the memory" , "Precisely on time" .)

I've seen this clue in the Evening Standard.

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