The most common and most likely answer to this crossword clue is the 11 letter word CONSTITUENT
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result CONSTITUENT
Buddy explains!

The straight part
We know that "Voter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case voter = constituent.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 CONSTITUENT mean?
CONSTITUENT noun- A part, or component of a whole
- A person or thing which constitutes, determines, or constructs
- A resident of an area represented by an elected official
- A voter of an area represented by an elected official
- A voter of a [political] candidate. A supporter of a cause
- One who appoints another to act for him as attorney in fact
- (grammar) A functional element of a phrase or clause
- Being a part, or component of a whole
- Authorized to make a constitution