The most common and most likely answer to this crossword clue is the 3 plus 3 letter word SEA COW
I've seen this crossword clue in The Daily Telegraph
S
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A
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More clues leading to the result SEA COW
Buddy explains!
The straight part
We know that "Main milk supplier?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main milk supplier? = sea cow.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 SEA COW mean?
SEA COW noun- Any of several marine mammals of the order Sirenia, including the manatee and dugong.
- Hippopotamus.