As preserves, they're there for the opening (4)
I believe the answer is:
tins
'as preserves' is the definition.
'tins' can be an answer for 'preserves' (tinning is a kind of preserving). I am not sure about the 'as' bit.
'there for the opening' is the wordplay.
'there' becomes 'ins' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' means one lot of letters go next to another (I've seen this in other clues).
'opening' indicates taking the first letters.
The first letter of 'the' is 't'.
'ins' put after 't' is 'TINS'.
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for tins that I've seen before include "food containers" , "Cans" .)