A short one? By no means! (3,2,3)
I believe the answer is:
not at all
'by no means' is the definition.
(I've seen this before)
'a short one?' is the wordplay.
I cannot quite understand how this works, but
'a' is found in the answer.
'one?' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for not at all that I've seen before include "By no means - polite disclaimer" , "Never!" , "You're welcome" , "Don't mention it!" , "acceptance of apology" .)