Unnaturally large shadow, wildly berserk concept (7,7)

I believe the answer is:

brocken spectre

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'unnaturally large shadow' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'wildly berserk concept' is the wordplay.
'wildly' indicates anagramming the letters.
'berserk'+'concept'='berserkconcept'
'berserkconcept' is an anagram of 'BROCKEN SPECTRE'.

Can you help me to learn more?

(Other definitions for brocken spectre that I've seen before include "Shadow effect seen in high mountains" , "nebulous image for peak viewing" .)

I've seen this clue in The Guardian.

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