The most common and most likely answer to this crossword clue is the 9 plus 4 letter word CONDUCTED TOUR
I've seen this crossword clue in The Times
C
O
N
D
U
C
T
E
D
T
O
U
R
Buddy explains!
The straight part
We know that "sightseeing trip" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sightseeing trip = conducted tour.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.