Test, for instance twice (5)
I believe the answer is:
assay
'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" .)