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 certain of the 'as' bit.
'there for the opening' is the wordplay.
'there' becomes 'ins' (I am not sure about this - if you are sure you should believe this answer much more).
'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' 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" .)