The most common and most likely answer to this crossword clue is the 13 letter word WHISTLE-STOPS
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "campaigning politician’s itinerary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case campaigning politician’s itinerary = whistle-stops.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.
Crossword Answer definitions
WHISTLE-STOPS noun- A small train station.
- A brief stop on a campaign tour to make a speech or perform some other type of public electioneering.