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'm not sure about the 'as' bit.
'there for the opening' is the wordplay.
'there' becomes 'ins' (I can't explain this - if you can you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'opening' indicates taking the first letters.
The initial letter of 'the' is 't'.
'ins' put after 't' is 'TINS'.
'they're' is the link.
Can you help me to learn more?
(Other definitions for tins that I've seen before include "food containers" , "Cans" .)