Crossword Genius

Must let its reviews show a measure of acidity (6,4)

Ross

I believe the answer is:

litmus test

I'm a little stuck... Click here to teach me more about this clue!

'a measure of acidity' is the definition.
I can't judge whether this definition defines the answer.

'must let its reviews' is the wordplay.
'reviews' indicates anagramming the letters (I've seen 'reviewed' mean this (letters reviewed or revised)).
'must'+'let'+'its'='mustletits'
'mustletits' is an anagram of 'LITMUS TEST'.

'show' acts as a link.

Can you help me to learn more?

(Other definitions for litmus test that I've seen before include "Observer may see red after one" , "Chemical trial" .)

I've seen this clue in the Sydney Morning Herald.
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