Excellent stand-in welcomes training run (6)
I believe the answer is:
superb
'excellent' is the definition.
(I've seen this before)
'stand-in welcomes training run' is the wordplay.
'stand in' becomes 'sub' (synonyms).
'welcomes' is an insertion indicator (in sense of 'invites in').
'training' becomes 'PE' (Physical Education).
'run' becomes 'r' (cricket abbreviation).
'pe'+'r'='per'
'sub' placed around 'per' is 'SUPERB'.
(Other definitions for superb that I've seen before include "Dazzling" , "fine" , "Highly impressive" , "choice" , "Surpassingly good" .)