The solver must respond with an answer? How ridiculous!
I believe the answer is:
i ask you
'answer? how ridiculous' is the definition.
I can't tell whether this defines the answer.
'the solver must respond with an' is the wordplay.
I cannot really understand how this works, but
'solver' could be 'you' (the person solving the crossword) and 'you' is found in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is located in the answer.
The remaining letters 'ask' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for i ask you that I've seen before include "Exclamation of annoyance" , "how ridiculous is that?" , "Expression of criticism" , "that's hard to credit" , "Heavens" .)