It's funny how zero crops up in Roman numerals! (5)
I believe the answer is:
comic
'it's funny' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for comic that I've seen before include "Funny (person, paper)" , "Garden perhaps" , "magazine for kids" , "publication for children" , "Comedian - humorous - children''s magazine" .)