The most common and most likely answer to this crossword clue is the 8 letter word GUERNSEY
I've seen this crossword clue in The Daily Telegraph
G
U
E
R
N
S
E
Y
More clues leading to the result GUERNSEY
- Man apprehending sneer changed sweater
- Gurney’s abroad, touring eastern island?
- Source of milk may provide us with energy
- Island where guys ne’er break down?
- Woolly cow
- Milk producer gives us energy another way
- Source of milk that may provide us with energy
- After knitting use new grey jumper
- Island that could give us energy supply
- Jersey relative - man suppressing crooked sneer
Buddy explains!

The straight part
We know that "Source of milk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of milk = guernsey.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 GUERNSEY mean?
GUERNSEY noun- A seaman's knitted woolen sweater, similar to a jersey.
- The shirt worn by the players.
- (slang: as in "get a guernsey") [receive] praise, admiration, recognition, credit, etc