Splendid Welsh league accepting representation of founder (9)
I believe the answer is:
wonderful
'splendid' is the definition.
(thesaurus)
'welsh league accepting representation of founder' is the wordplay.
'welsh' becomes 'W' (abbreviation).
'league' becomes 'l' (abbreviation).
'accepting' indicates putting letters inside (inserted letters are accepted).
'representation of' indicates anagramming the letters (I've seen this in other clues).
'w'+'l'='wl'
'founder' anagrammed gives 'onderfu'.
'wl' placed around 'onderfu' is 'WONDERFUL'.
(Other definitions for wonderful that I've seen before include "Awe-inspiring" , "Great" , "Marvellous" , "Fantastic, terrific" , "Superb" .)