Answers that one can make a pile in the reserves! (7)
I believe the answer is:
replies
'answers that one' is the definition.
'replies' can be an answer for 'answers' ('reply' can be a synonym of 'answer'). I am unsure of the remainder of the definition.
'make a pile in the reserves' is the wordplay.
'make' is an anagram indicator.
'in' means one lot of letters goes inside another.
'the reserves' becomes 'res' (I can't justify this - if you can you should give a lot more credence to this answer).
'pile' is an anagram of 'plie'.
'plie' put inside 'res' is 'REPLIES'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for replies that I've seen before include "Reacts verbally" , "Answers" , "Responds" .)