Proficient testament to benevolence (8)
I believe the answer is:
goodwill
'benevolence' is the definition.
(I've seen this before)
'proficient testament' is the wordplay.
'proficient' becomes 'good' (synonyms).
'testament' becomes 'will' (synonyms).
'good'+'will'='GOODWILL'
'to' is the link.
(Other definitions for goodwill that I've seen before include "friendly attitude" , "Benevolence" , "intangible asset" , "Disposition to kindness" , "compassion" .)