Jam ingredient originally exported in copper container (6)
I believe the answer is:
pectin
'jam ingredient' is the definition.
(I've seen this in another clue)
'originally exported in copper container' is the wordplay.
'originally' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'copper' becomes 'PC' (police constable).
'container' becomes 'tin' (tin is a kind of container).
The first letter of 'exported' is 'e'.
'e' put inside 'pc' is 'pec'.
'pec'+'tin'='PECTIN'
(Other definitions for pectin that I've seen before include "Jelly ingredient" , "Gelatinous substance in ripefruits, makes jam set" , "something useful for setting?" , "could be found in jam" , "Carbohydrate used in making fruit jellies and jams" .)