Rigorous trials start to assess Policemen's Ball after last one's cancelled (4,5)
I believe the answer is:
acid tests
'rigorous trials' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Perhaps you can see an association between them that I don't see?
'start to assess policemen's ball after last one's cancelled' is the wordplay.
I cannot really understand how this works, but
'assess' could be 'test' (I've seen this before) and 'test' is located in the answer.
'policemen's' could be 'cids' (abbreviation for Criminal Investigation Department) and 'cids' is present in the leftover letters.
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for acid tests that I've seen is " Rigorous or crucial appraisals".)