The most common and most likely answer to this crossword clue is the 8 plus 4 letter word MARGINAL SEAT
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More clues leading to the result MARGINAL SEAT
- On the outside an arse, not one to be taken lightly
- French victim of assassination pierced by a single shot — political target?
- Constituency in the Borders?
- It is hard to defend profit — at least most of it is wrong
- Wandering emigrant, alas, political target?
- Does such a constituency have MPs on edge?
- Border constituency? Not necessarily
Buddy explains!
The straight part
We know that "not one to be taken lightly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not one to be taken lightly = marginal seat.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.