Brilliant material rejected in advance (6)
I believe the answer is:
superb
'brilliant' is the definition.
(thesaurus)
'material rejected in advance' is the wordplay.
'material' becomes 'rep' (rep is a kind of material**).
'rejected' is a reversal indicator.
'in' indicates putting letters inside.
'advance' becomes 'sub' (I've seen this in other clues).
'rep' back-to-front is 'per'.
'per' placed within 'sub' is 'SUPERB'.
(Other definitions for superb that I've seen before include "Highly impressive" , "Surpassingly good" , "First class" , "Impressively splendid" , "'Excellent, brilliant (6)'" .)