Brief indulgence certainly something highly valued (8)
I believe the answer is:
treasure
'something highly valued' is the definition.
I can't tell whether this definition defines the answer.
'brief indulgence certainly' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'indulgence' becomes 'treat' (to indulge yourself is to treat yourself).
'certainly' becomes 'sure' (synonyms).
'treat' with its last letter taken away is 'trea'.
'trea'+'sure'='TREASURE'
Can you help me to learn more?
(Other definitions for treasure that I've seen before include "Cherish like heap of valuables" , "Highly-prize a possession" , "Stored-up wealth" , "Value greatly" , "much-loved person" .)