Childishly immature (9)
I believe the answer is:
infantile
(Other definitions for infantile that I've seen before include "in early stage of development" , "for kids" , "Babyish" , "Inflate in childish way" .)
I've seen this clue in The Mirror and The Telegraph.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free.
Check out my app
or
learn more
about the Crossword Genius project.