Crossword Genius

Test, for instance twice (5)

Ross

I believe the answer is:

assay

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'test' is the definition.
(I know that assay is a type of test)

'for instance twice' is the wordplay.
'for instance' becomes 'as'.
'twice' becomes 'say' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'as'+'say'='ASSAY'

Can you help me to learn more?

(Other definitions for assay that I've seen before include "Test ore for value" , "Trial" , "'Analyse, precious metal, say (5)'" , "Put to the proof" , "Evaluate" .)

I've seen this clue in The Times.
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