The most common and most likely answer to this crossword clue is the 13 letter word MATERIALISING
I've seen this crossword clue in Financial Times
M
A
T
E
R
I
A
L
I
S
I
N
G
Buddy explains!

The straight part
We know that "Making appearance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case making appearance = materialising.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 MATERIALISING mean?
MATERIALISING verb- To cause to take physical form, or to cause an object to appear.
- To take physical form, to appear seemingly from nowhere.
- To regard as matter; to consider or explain by the laws or principles which are appropriate to matter.