The most common and most likely answer to this crossword clue is the 6 letter word UDDERS
I've seen this crossword clue in The Guardian
U
D
D
E
R
S
Buddy explains!

The straight part
We know that "Suppliers of milk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suppliers of milk = udders.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 UDDERS mean?
UDDERS noun- An organ formed of the mammary glands of female quadruped mammals, particularly ruminants such as cattle, goats, sheep and deer.
- (impolite) A woman's breast.