The most common and most likely answer to this crossword clue is the 3 plus 4 letter word GET AWAY
I've seen this crossword clue in The Sunday Telegraph
G
E
T
A
W
A
Y
Buddy explains!

The straight part
We know that "Take a holiday/That seems unlikely" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case take a holiday/that seems unlikely = get away.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.
Crossword Answer definitions
GET AWAY adjective- Pertaining to an escape, as in a vehicle or plans.