Marx discussing royalty, the main killer (9)

I believe the answer is:

harpooner

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'main killer' is the definition.
The definition suggests a singular noun which matches the answer.

'marx discussing royalty' is the wordplay.
'marx' becomes 'harpo' (Harpo Marx).
'discussing royalty' becomes 'oner' (I can't explain this - if you can you should believe this answer much more).
'harpo'+'oner'='HARPOONER'

'the' acts as a link.

Can you help me to learn more?

(Other definitions for harpooner that I've seen before include "killer at sea" , "I do for whales" .)

I've seen this clue in The Times.

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