Great Britain's fancy clothing lines (9)
I believe the answer is:
brilliant
'great' is the definition.
(I know that great can be written as brilliant)
'britain's fancy clothing lines' is the wordplay.
'fancy' is an anagram indicator.
'clothing' indicates putting letters inside (some letters cover up or clothe others).
'lines' becomes 'll' (l is an abbreviation for line).
'britain' with letters rearranged gives 'briiant'.
'briiant' enclosing 'll' is 'BRILLIANT'.
(Other definitions for brilliant that I've seen before include "Glittering" , "Shining intensely" , "Very bright or exceptionally talented" , "Exceptionally clever" , "'Wonderful, dazzling (9)'" .)