Not in one's own interest (8)
I believe the answer is:
selfless
'not in' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'one's own interest' is the wordplay.
'one's own' becomes 'self'.
'interest' becomes 'less' (I can't justify this - if you can you should give a lot more credence to this answer).
'self'+'less'='SELFLESS'
Can you help me to learn more?
(Other definitions for selfless that I've seen before include "Considerate" , "Generous" , "Altruistic" .)