British want jet (5)
I believe the answer is:
black
'jet' is the definition.
(similar in meaning)
'british want' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'want' becomes 'lack' (synonyms).
'b'+'lack'='BLACK'
(Other definitions for black that I've seen before include "Colour of jet" , "Very darkest colour" , "Gloomy" , "Served without milk" , "-- Monday" .)