Safeguards stockpile in addition to mail (9)
I believe the answer is:
preserves
'mail' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'safeguards stockpile in addition' is the wordplay.
'safeguards stockpile' becomes 'reserve' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'addition' becomes 'ps'.
'reserve' going into 'ps' is 'PRESERVES'.
'to' is the link.
Can you help me to learn more?
(Other definitions for preserves that I've seen before include "Stores - pickles" , "Keeps in unchanged condition, or from decay" , "Keeps in unaltered condition" , "Keeps the jams" , "Items sold at school fetes" .)