The most common and most likely answer to this crossword clue is the 11 letter word RESERVATION
I've seen this crossword clue in The Times
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More clues leading to the result RESERVATION
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The straight part
We know that "Advance booking/condition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case advance booking/condition = reservation.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 RESERVATION mean?
RESERVATION noun- The act of reserving, withholding or keeping back.
- Something that is withheld or kept back.
- (often in the plural) A limiting qualification; a doubt.
- A tract of land set apart by the US government for the use of a Native American people; Indian reservation (compare Canadian reserve).
- An arrangement by which accommodation or transport arrangements are secured in advance.
- The area which separates opposing lanes of traffic on a divided motorway or dual carriageway; see also central reservation.
- The setting aside of a certain percentage of vacancies in government institutions for members of backward and underrepresented communities (defined primarily by caste and tribe).