Outstanding lakes in Britain polluted (9)
I believe the answer is:
brilliant
'outstanding' is the definition.
(thesaurus)
'lakes in britain polluted' is the wordplay.
'lakes' becomes 'LL' (a geographical abbreviation for lake written twice).
'in' means one lot of letters goes inside another.
'polluted' is an anagram indicator.
'britain' is an anagram of 'briiant'.
'll' put inside 'briiant' is 'BRILLIANT'.
(Other definitions for brilliant that I've seen before include "Of outstanding ability" , "Very bright or exceptionally talented" , "'Wonderful, dazzling (9)'" , "Fantastic!" , "Of surpassing excellence" .)