The most common and most likely answer to this crossword clue is the 12 letter word OFFICEHOLDER
I've seen this crossword clue in The Times
O
F
F
I
C
E
H
O
L
D
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Buddy explains!
The straight part
We know that "Incumbent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case incumbent = officeholder.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OFFICEHOLDER mean?
OFFICEHOLDER noun- A person who holds an office, especially one appointed or elected to a public office; an incumbent