Reply unexpectedly, with solution to defend (6,4)
I believe the answer is:
answer back
'reply unexpectedly' is the definition.
'answer back' can be an answer for 'reply' (answering back is a kind of replying). I'm not sure about the 'unexpectedly' bit.
'with solution to defend' is the wordplay.
'with' means one lot of letters go next to another.
'solution' becomes 'answer' (synonyms).
'to defend' becomes 'back' (I can't explain this - if you can you should believe this answer much more).
'answer' next to 'back' is 'ANSWER BACK'.
Can you help me to learn more?
(Other definitions for answer back that I've seen before include "Reply pertly" , "reply impertinently" , "Retort" , "Be impertinent" .)