Judge film — British want something extremely dark (3,5)
I believe the answer is:
jet black
'extremely dark' is the definition.
'judge film british want something' is the wordplay.
'judge' becomes 'j'.
'film' becomes 'et'.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'want something' becomes 'lack'.
'j'+'et'+'b'+'lack'='JET-BLACK'
(Other definitions for jet black that I've seen before include "Very darkest colour" , "very dark prospect" , "Glossy and very dark" .)