Difficult to put nothing in a stockpile! (5)
I believe the answer is:
hoard
'a stockpile' is the definition.
(I've seen this before)
'difficult to put nothing in' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'to put nothing' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'hard' placed around 'o' is 'HOARD'.
Can you help me to learn more?
(Other definitions for hoard that I've seen before include "Store of money etc, often hidden" , "Rhoda (anag.)" , "Valuable store" , "A lot of people" , "Store, usually hidden" .)