Natural should, once trained (10)

I believe the answer is:

unschooled

'natural' is the definition.
(thesaurus)

'should once trained' is the wordplay.
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'should'+'once'='shouldonce'
'shouldonce' is an anagram of 'UNSCHOOLED'.

(Another definition for unschooled that I've seen is " Lacking formal education".)

I've seen this clue in The Telegraph.

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