Harking back to the past in cabaret routine (5)

I believe the answer is:

retro

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'harking back to the past' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for retro that I've seen before include "Type of rocket; backward-looking" , "Backward looking" , "Looking backwards" , "Prefix for old-style" , "recapturing the past" .)

I've seen this clue in The Sunday Times and The Times.

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