Excellent rescue dog detailed to enter clubs twice (7)
I believe the answer is:
classic
'excellent' is the definition.
(thesaurus)
'rescue dog detailed to enter clubs twice' 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' is an insertion indicator.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'twice' says to double the letters.
'lassie' with its last letter taken off is 'lassi'.
'c' doubled is 'cc'.
'lassi' inserted within 'cc' is 'CLASSIC'.
(Other definitions for classic that I've seen before include "Work of enduring excellance" , "Epsom Derby or St Leger Stakes, for example" , "Descriptive of ancient Rome and Greece" , "Oaks perhaps" , "Having qualities like those of acknowledged art" .)