One takes the biscuit, as against a donkey? (11)
I believe the answer is:
crackerjack
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'the biscuit as against a donkey?' is the wordplay.
'the biscuit as' becomes 'cracker' (I am not sure about this - if you are sure you should believe this answer much more).
'against' means one lot of letters go next to another.
'a donkey?' becomes 'jack' (I have seen 'Male donkey ' mean 'jack' so perhaps 'donkey' could also mean 'jack').
'cracker'+'jack'='CRACKERJACK'
'takes' is the link.
(Other definitions for crackerjack that I've seen before include "Wonderful (person or thing)" , "Excellent (person or thing)" , "Exceptionally good" , "Exceptional (thing or person) - 1970s' children's programme" .)