The most common and most likely answer to this crossword clue is the 8 letter word PREORDER
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The straight part
We know that "reserve in advance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reserve in advance = preorder.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 PREORDER mean?
PREORDER noun- An order for goods or services placed in advance.
- A binary relation that is reflexive and transitive.
- To order (goods or services) in advance, before they are available.
- To sort or arrange beforehand.
- (of a traversal of a tree) Such that, recursively, the root is visited before the left and right subtrees.