They fight invaders who libelled Scot after Reformation (5,5,5)

I believe the answer is:

white blood cells

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'they fight invaders' is the definition.
The definition suggests an adverb but the answer is not.

'who libelled scot after reformation' is the wordplay.
'after reformation' indicates an anagram.
'who'+'libelled'+'scot'='wholibelledscot'
'wholibelledscot' is an anagram of 'WHITE BLOOD CELLS'.

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I've seen this clue in The Guardian.

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