The most common and most likely answer to this crossword clue is the 10 letter word LOGGERHEAD
I've seen this crossword clue in The Guardian
L
O
G
G
E
R
H
E
A
D
Buddy explains!

The straight part
We know that "turtle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case turtle = loggerhead.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 LOGGERHEAD mean?
LOGGERHEAD noun- A stupid person; a blockhead, a dolt.
- A metal tool consisting of a long rod with a bulbous end that is made hot in a fire, then plunged into some material (such as pitch or a liquid) to melt or heat it.
- A post on a whaling boat used to secure the harpoon rope.
- (often in the plural) A thistle-like flowering plant of the genus Centaurea, particularly the common knapweed (Centaurea nigra).
- Used as the name of various animals with large heads.