The most common and most likely answer to this crossword clue is the 10 letter word DISCONNECT
I've seen this crossword clue in The Guardian
D
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Buddy explains!
The straight part
We know that "Separate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case separate = disconnect.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 DISCONNECT mean?
DISCONNECT noun- A break or interruption in an existing connection, continuum, or process; disconnection.
- A switch used to isolate a portion of an electrical circuit.
- A lack of connection or accord; a mismatch.
- The deliberate severing of ties with family, friends, etc. considered antagonistic towards Scientology.
- To sever or interrupt a connection.
- Of a person, to become detached or withdrawn.
- To remove the connection between an appliance and an electrical power source.