In a childish manner (10)

I think it could be either:

childishly

immaturely

'in a childish manner' is the definition.
(I know this)

This is the entire clue.

(Other definitions for childishly that I've seen before include "in puerile manner" , "In an immature fashion" , "Like baby" .)

I've seen this clue in The Times.

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