The most common and most likely answer to this crossword clue is the 2 plus 4 plus 2 letter word AS WELL AS
I've seen this crossword clue in Independent
A
S
W
E
L
L
A
S
More clues leading to the result AS WELL AS
Buddy explains!

The straight part
We know that "In addition to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in addition to = as well as.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.