Crucial evidence for experimental diets, indeed (4,4)
I believe the answer is:
acid test
'crucial evidence for experimental' is the definition.
I can't judge whether this defines the answer.
'diets indeed' is the wordplay.
I cannot quite see how this works, but
an anagram of 'diets' is 'idtes' which is within the answer.
The remaining letters 'act' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for acid test that I've seen before include "Severe or conclusive examination" , "A severe or conclusive exam or trial" , "'Appraisal to show the true value (4,4)'" , "Validity confirmed" , "Severe trial; old assay method" .)