Splendid police officer, British (6)
I believe the answer is:
superb
'splendid' is the definition.
(thesaurus)
'police officer british' is the wordplay.
'police officer' becomes 'super' (term for a senior police officer).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'super'+'b'='SUPERB'
(Other definitions for superb that I've seen before include "Impressively splendid" , "Quite splendid, really excellent" , "'Excellent, brilliant (6)'" , "Brilliant, fantastic" , "First-class" .)