Helping shop to stock newspaper (6)
I believe the answer is:
ration
'helping' is the definition.
(a ration is a serving or helping)
'shop to stock newspaper' is the wordplay.
'shop' becomes 'rat on' (both can mean to betray someone).
'to stock' indicates putting letters inside (some letters contain or stock others).
'newspaper' becomes 'i' (UK newspaper).
'raton' enclosing 'i' is 'RATION'.
(Other definitions for ration that I've seen before include "Parcel out scarce food, say" , "Restricted share" , "Food allowance" , "Wartime allowance?" , "Limit quantity as of food during scarcity" .)