Most immature through reversing in retreat (7)

I believe the answer is:

naivest

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'most immature' is the definition.
('naive' can be similar in meaning to 'immature')

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

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(Other definitions for naivest that I've seen before include "very green" , "far from the most wise?" , "Most unsophisticated" , "Most open" , "Most simple" .)

I've seen this clue in The Telegraph.

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