12 across
Replacement
notices
changed
as
one
result
of
twice
dividing
(10)
Replacement notices changed as one result of twice dividing (10)
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The definition is at the end of the clue.
How the clue works
One result of twice dividing is your definition, since split something twice and you've got subsections. The wordplay has SUB meaning "replacement," followed by an anagram of NOTICES (that's your "changed") giving SECTION, and the two parts slot together neatly.