Toughest horrify for a full two days (5-5,5)

I believe the answer is:

forty-eight hours

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'a full two days' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'toughest horrify' is the wordplay.
'toughest horrify' anagrams to 'FORTY EIGHT HOURS'.
However, I am unsure how the anagram is indicated.

'for' acts as a link.

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

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