The most common and most likely answer to this crossword clue is the 12 letter word TRANSVESTITE
I've seen this crossword clue in The Guardian
T
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Buddy explains!
The straight part
We know that "old cross-dresser" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old cross-dresser = transvestite.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 TRANSVESTITE mean?
TRANSVESTITE noun- A person who sometimes wears clothes traditionally worn by and associated with the opposite sex; typically a male who cross-dresses occasionally by habit or personal choice.
- A person, typically a heterosexual male, who compulsively seeks and derives paraphilic sexual arousal from cross-dressing, especially if the urges and behavior cause the patient distress or social impairment.