Cardinal's cape left by primate in Rome (7)
I believe the answer is:
hundred
'cardinal's' is the definition.
(similar in meaning)
'left by primate in rome' is the wordplay.
'left' becomes 'red' (red can informally mean communist).
'by' means one lot of letters go next to another.
'primate in rome' becomes 'hund' (I can't explain this - if you can you should give a lot more credence to this answer).
'red' put after 'hund' is 'HUNDRED'.
'cape' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for hundred that I've seen before include "Old part of county" , "Old division of a county" , "entirely associated with 27 [PERCENT]" , "Number - old part of a shire" , "C in Roman numerals" .)