Excellent exercise runs in advance (6)
I believe the answer is:
superb
'excellent' is the definition.
(thesaurus)
'exercise runs in advance' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'runs' becomes 'r' (cricket abbreviation).
'in' indicates putting letters inside.
'advance' becomes 'sub' (I've seen this in other clues).
'pe'+'r'='per'
'per' put within 'sub' is 'SUPERB'.
(Other definitions for superb that I've seen before include "Surpassingly good" , "Terrific" , "First-class" , "Of surpassing excellence" , "Brilliant, fantastic" .)