The most common and most likely answer to this crossword clue is the 12 letter word PREREQUISITE
I've seen this crossword clue in The Guardian
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More clues leading to the result PREREQUISITE
Buddy explains!
The straight part
We know that "it must come first" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it must come first = prerequisite.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 PREREQUISITE mean?
PREREQUISITE noun- Something that must be gained in order to gain something else
- In education, a course or topic that must be completed before another course or topic can be started. May be colloquially referred to as a prereq.
- Required as a prior condition of something else; necessary or indispensable.