Except for the duck, it's a vegetarian affair (5)
I believe the answer is:
beano
'a vegetarian affair' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'except for the duck' is the wordplay.
'except for' becomes 'bean' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the duck' becomes 'o' (resembles zero - 'duck' in cricket).
'bean'+'o'='BEANO'
'it's' is the link.
Can you help me to learn more?
(Other definitions for beano that I've seen before include "Enjoyable party" , "Informal party" , "rowdy festivity" , "Junket - children's comic" , "Kids' comic" .)