The most common and most likely answer to this crossword clue is the 7 letter word OUTVOTE
I've seen this crossword clue in The Guardian
O
U
T
V
O
T
E
More clues leading to the result OUTVOTE
Buddy explains!
The straight part
We know that "defeat on polling day" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defeat on polling day = outvote.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 OUTVOTE mean?
OUTVOTE verb- To cast more votes than another
- To defeat another by obtaining more votes