Dart in hurriedly - exterior is theoretical and modern (14)

I believe the answer is:

nontraditional

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'modern' is the definition.
(synonyms)

'dart in hurriedly exterior is theoretical and' is the wordplay.
I cannot quite see how this works, but
an anagram of 'dart' is 'trad' which is within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
This explanation may well be incorrect...

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I've seen this clue in The Daily Express.

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