Conclusive proof of wild cat's diet (4,4)
I believe the answer is:
acid test
'conclusive proof' is the definition.
(I've seen this before)
'wild cat's diet' is the wordplay.
'wild' indicates an anagram.
'cats'+'diet'='catsdiet'
'catsdiet' anagrammed gives 'ACID TEST'.
'of' acts as a link.
(Other definitions for acid test that I've seen before include "crucial experiment" , "Cat's diet (anag.)" , "Validity confirmed" , "Rigorous or crucial appraisal" , "Conclusive proof" .)