Test size ingredients are the most piquant (8)

I believe the answer is:

zestiest

'the most piquant' is the definition.
(I've seen this in another clue)

'test size ingredients' is the wordplay.
'ingredients' is an anagram indicator.
'test'+'size'='testsize'
'testsize' anagrammed gives 'ZESTIEST'.

'are' is the link.

I've seen this clue in The Guardian.

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