The most common and most likely answer to this crossword clue is the 6 letter word NONFAT
I've seen this crossword clue in The Guardian
N
O
N
F
A
T
Buddy explains!
The straight part
We know that "Type of milk to make you slim" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case type of milk to make you slim = nonfat.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 NONFAT mean?
NONFAT adjective- Containing no fat; fat-free.