Must let its reviews show a measure of acidity (6,4)
I believe the answer is:
litmus test
'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" .)