The most common and most likely answer to this crossword clue is the 7 letter word RAIDING
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Buddy explains!

The straight part
We know that "on a sortie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on a sortie = raiding.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 RAIDING mean?
RAIDING verb- To engage in a raid against.
- To lure from another; to entice away from.
- To indulge oneself by taking from.
- The act of carrying out a raid.