Marx, remarkable person who spears his prey (9)
I believe the answer is:
harpooner
'who spears his prey' is the definition.
'marx remarkable person' is the wordplay.
'marx' becomes 'harpo' (Harpo Marx).
'remarkable person' becomes 'oner' (slang for a unique person or thing).
'harpo'+'oner'='HARPOONER'
(Other definitions for harpooner that I've seen before include "I do for whales" , "killer at sea" .)