The most common and most likely answer to this crossword clue is the 1 plus 3 plus 5 letter word A BAD APPLE
I've seen this crossword clue in The Times
A
B
A
D
A
P
P
L
E
Buddy explains!
The straight part
We know that "a corrupting influence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a corrupting influence = a bad apple.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.