Crazy starts not up to the young (5)

I believe the answer is:

nutty

'crazy' is the definition.
(I've seen this before)

'starts not up to the young' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'not up to the young' is 'NUTTY'.

(Other definitions for nutty that I've seen before include "Crunchy" , "Insane" , "Eccentric - like nougat?" , "like vegetarian roast?" , "like Brazil" .)

I've seen this clue in the Sydney Morning Herald.

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