An understanding of another's sadness (8)
I believe the answer is:
sympathy
'an understanding of another's sadness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sympathy that I've seen before include "Fellow-feeling, compassion" , "Understanding for the suffering of others" , "Feeling of tenderness and pity for sufferer" , "Fellow-feeling, understanding" , "Commiseration" .)