Decisive assessment essential in disputed titles (6,4)
I believe the answer is:
litmus test
'decisive assessment' is the definition.
'essential in disputed titles' is the wordplay.
'essential' becomes 'must' (similar in meaning).
'in' indicates putting letters inside.
'disputed' indicates anagramming the letters.
'titles' is an anagram of 'litest'.
'must' placed inside 'litest' is 'LITMUS TEST'.
(Other definitions for litmus test that I've seen before include "Chemical trial" , "Observer may see red after one" .)