Whaleman expert on dumb Marx brother (9)

I believe the answer is:

harpooner

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'dumb marx brother' is the definition.
The definition suggests a singular noun which matches the answer.

'whaleman expert on' is the wordplay.
'whaleman' becomes 'harpo' (I am not sure about this - if you are sure you should believe this answer much more).
'expert' becomes 'oner' (I've seen this before).
'on' says to put letters next to each other.
'harpo'+'oner'='HARPOONER'

Can you help me to learn more?

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

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