Cruel ambassador with no diplomatic skills (9)

I believe the answer is:

heartless

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'cruel' is the definition.
(similar in meaning)

'ambassador with no diplomatic skills' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'with no diplomatic skills' becomes 'artless' (I can't explain this - if you can you should give a lot more credence to this answer).
'he'+'artless'='HEARTLESS'

Can you help me to learn more?

(Other definitions for heartless that I've seen before include "Completely lacking in feeling" , "Cruel and unfeeling" , "That's very cruel" , "Without pity or feeling, being defective in cardiac way" , "Lacking pity due to cardiac deficiency" .)

I've seen this clue in The Guardian.

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