Catholic Celtic played boring European clubs (8)
I believe the answer is:
eclectic
'catholic' is the definition.
(thesaurus)
'celtic played boring european clubs' is the wordplay.
'played' indicates an anagram.
'boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'european' becomes 'e' (abbreviation e.g. EU).
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'celtic' is an anagram of 'clecti'.
'e'+'c'='ec'
'clecti' put inside 'ec' is 'ECLECTIC'.
(Other definitions for eclectic that I've seen before include "Choosing or accepting ideas etc from many sources" , "Selecting from various sources" , "Deriving ideas from many sources" , "Choosing ideas or beliefs from various sources" , "Discriminating" .)