A British opera, length unusual (8)
I believe the answer is:
abnormal
'unusual' is the definition.
(similar in meaning)
'a british opera length' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'opera' becomes 'norma' (I've seen this in another clue).
'length' becomes 'l' (abbreviation in geometry).
'a'+'b'+'norma'+'l'='ABNORMAL'
(Other definitions for abnormal that I've seen before include "Non-standard" , "Exceptional; strange" , "Irregular" , "Odd" , "Deviating from the standard" .)