Help explain this clue: "Choose a filling for pastries"

This is from Games World of Puzzles, February 2024. The puzzle is "Beginnings and Endings" by Patrick Berry.
The clue is:
Choose a filling for pastries (Hyph.)
and the answer is:
POP-TARTS
The explanation given is:
(opt + parts)
I see how OPT means "choose", and POP-TARTS are "pastries", but I don't see how "a filling for" can give both PARTS and an anagram or container indicator. "Filling" could be a container indicator, of course, but that would mean that "for" would have to be used to define PARTS.
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Joshua Kreitzer
gromit82@hotmail.com