The cryptic clue "51 +cn fun" likely involves wordplay where "51" represents the Roman numerals LI (50 = L, 1 = I). Adding "cn" (the letters C and N) gives the letters L, I, C, N. These can be rearranged to form part of the word "CARNIVAL", which is a fun event. While the wordplay isn't direct, "carnival" incorporates the letters from LI and CN and aligns with the definition "fun".
Answer: CARNIVAL
