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