Being Red, set out to imitate Marx (7)
I believe the answer is:
scarlet
'being red' is the definition.
(synonyms)
'set out to imitate marx' is the wordplay.
'out' means one lot of letters goes inside another.
'to imitate marx' becomes 'carl' (I can't justify this - if you can you should give a lot more credence to this answer).
'set' going around 'carl' is 'SCARLET'.
Can you help me to learn more?
(Other definitions for scarlet that I've seen before include "Clarets (anag.)" , "Murderer" , "Shade of red" , "Crimson" , "Vivid red" .)