Belief in pleasure as the highest good (8)
I believe the answer is:
hedonism
'belief in pleasure as the highest good' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hedonism that I've seen before include "The pursuit of happiness as a matter of principle" , "System based on pleasure as the highest good" , "The pursuit of and devotion to pleasure" , "The pusuit of pleasure as life's greatest good" , "The belief that pleasure is the chief good in life" .)