Brilliant rescue dog detailed to enter cricket club (7)
I believe the answer is:
classic
'brilliant' is the definition.
'rescue dog detailed to enter cricket club' is the wordplay.
'rescue dog' becomes 'lassie'.
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'to enter' indicates putting letters inside.
'cricket club' becomes 'cc'.
'lassie' with its last letter taken away is 'lassi'.
'lassi' put within 'cc' is 'CLASSIC'.
(Other definitions for classic that I've seen before include "Simple, neat and elegant" , "work of the highest quality" , "Recognised as excellent" , "Race, eg Derby" , "Of recognized excellence" .)