Adult packed wrong pants (4-5)

I believe the answer is:

full-grown

'adult' is the definition.
(I know that adult can be written as full-grown)

'packed wrong pants' is the wordplay.
'packed' becomes 'full' (synonyms).
'pants' is an anagram indicator (UK informal term for 'nonsense' or 'rubbish').
'wrong' anagrammed gives 'grown'.
'full'+'grown'='FULL-GROWN'

(Another definition for full-grown that I've seen is " Adult".)

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