The most common and most likely answer to this crossword clue is the 9 letter word IMAGINARY
I've seen this crossword clue in Financial Times
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More clues leading to the result IMAGINARY
- One contrary girl swallowing a drink? It's not true
- Pretend one woman’s astride another
- Pretend Wanderer bumps into Madonna, holding a drink
- Pretend writer’s against change with no leadership
- First girl to swallow gallon in one? Fabulous!
- Unreal yarn I concocted, taking wise men in
- Pretend I am angry with one suspect
- Made up by a girl over a drink with Poins
- I’m getting old, short and fairly regularly ignored — it’s not real!
- The setter’s ace trap — a set of lines that’s not real
Buddy explains!

The straight part
We know that "Pretend" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pretend = imaginary.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 IMAGINARY mean?
IMAGINARY noun- Imagination; fancy.
- An imaginary quantity.
- The set of values, institutions, laws, and symbols common to a particular social group and the corresponding society through which people imagine their social whole.
- Existing only in the imagination.
- (of a number) Having no real part; that part of a complex number which is a multiple of the square root of -1.