Just when pearl of wisdom can be testing? (8)
I believe the answer is:
Here is my best explanation:
'be testing?' is the definition.
'just when pearl of wisdom' is the subsidiary indication.
'just when' becomes 'as'.
'pearl of wisdom' becomes 'saying'.
'can' is the link.
(Other definitions for assaying that I've seen before include "Testing", "Evaluating", "Analysing to determine properties", "Analysing the composition of metal" and "Analyzing to determine properties".)