British have no charm: this is diabolical (5,5)
I believe the answer is:
black magic
'this is diabolical' is the definition.
The definition suggests a singular noun which matches the answer.
'british have no charm' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'have no' becomes 'lack' (to lack something is to have none of it).
'charm' becomes 'magic' (I can't explain this - if you can you should give a lot more credence to this answer).
'b'+'lack'+'magic'='BLACK MAGIC'
Can you help me to learn more?
(Other definitions for black magic that I've seen before include "Dangerous spells" , "The dark arts" , "Evil use of supernatural power" , "Evil sorcery" , "Wizardry for evil purposes" .)