Museum tour starting from old city at the end of June is not fully developed (8)

I believe the answer is:

immature

I'm a little stuck... Click here to teach me more about this clue!

'not fully developed' 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 immature that I've seen before include "Irate mum? (anag)" , "Lacking in development" , "Underdeveloped - childish" , "Young" , "behaviour of child" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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