Whaleman expert on dumb Marx brother (9)
I believe the answer is:
harpooner
'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 give a lot more credence to this answer).
'expert' becomes 'oner' (I've seen this before).
'on' means one lot of letters go next to another.
'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" .)