Aid in wrongdoing putting a duck in batter? Rubbish! (7)
I believe the answer is:
abettor
'aid in wrongdoing' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'a duck in batter? rubbish' is the wordplay.
'a duck' becomes 'o' (resembles zero - 'duck' in cricket).
'in' means one lot of letters goes inside another.
'rubbish' indicates an anagram (a poor quality or rubbish spelling of the letters).
'batter' with letters rearranged gives 'abettr'.
'o' placed into 'abettr' is 'ABETTOR'.
'putting' acts as a link.
Can you help me to learn more?
(Other definitions for abettor that I've seen before include "fellow-criminal" , "Associate" , "one to encourage or help" , "Assistant" , "One who helps" .)