Just when pearl of wisdom can be testing? (8)
I believe the answer is:
assaying
'be testing?' is the definition.
(I've seen this before)
'just when pearl of wisdom' is the wordplay.
'just when' becomes 'as'.
'pearl of wisdom' becomes 'saying'.
'as'+'saying'='ASSAYING'
'can' acts as a link.
(Other definitions for assaying that I've seen before include "Testing" , "Evaluating" , "Analysing to determine properties" , "Analysing the composition of metal" , "Analyzing to determine properties" .)