The most common and most likely answer to this crossword clue is the 7 letter word CREAMER
I've seen this crossword clue in Financial Times
C
R
E
A
M
E
R
Buddy explains!

The straight part
We know that "white powder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case white powder = creamer.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 CREAMER mean?
CREAMER noun- A jug for holding cream.
- A separator for removing cream from milk to leave skimmed milk.
- A nondairy product that adds a creamy texture to coffee.
- A small thimble sized container of milk or cream, used as a standard individual "cream" or "milk" portion, typically provided by restaurants for cups of tea or coffee.