Key indicator in case covering appeal: a necessity (6,4)
I believe the answer is:
litmus test
'key indicator in case covering appeal a necessity' is the definition.
I can't tell whether this definition defines the answer.
'in case covering appeal a necessity' is the wordplay.
'in case' becomes 'lest' (I've seen this in another clue).
'covering' means one lot of letters goes inside another.
'appeal' becomes 'it' ('it' can mean something attractive or appealing).
'a necessity' becomes 'must' (I've seen this in another clue).
'it'+'must'='itmust'
'lest' placed around 'itmust' is 'LITMUS TEST'.
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" .)