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' means one lot of letters goes inside another (inserted letters are accepted).
'representation of' indicates an anagram (I've seen this in other clues).
'w'+'l'='wl'
'founder' is an anagram of 'onderfu'.
'wl' enclosing 'onderfu' is 'WONDERFUL'.
(Other definitions for wonderful that I've seen before include "Awe-inspiring" , "Great" , "Marvellous" , "Fantastic, terrific" , "Superb" .)