Flat British countryside (5)
I believe the answer is:
bland
'flat' is the definition.
(similar in meaning)
'british countryside' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'countryside' becomes 'land' (I've seen this in other clues).
'b'+'land'='BLAND'
(Other definitions for bland that I've seen before include "Insipid; lacklustre" , "Suave" , "Unruffled but unstimulating" , "Insipid, vapid" , "Mild; dull" .)