The most common and most likely answer to this crossword clue is the 8 letter word FREUDIAN
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The straight part
We know that "Slip" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slip = freudian.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 FREUDIAN mean?
FREUDIAN adjective- Of or relating to Austrian neurologist Sigmund Freud's scientific theory and psychotherapy called psychoanalysis.
- Relating to or influenced by the Austrian neurologist Sigmund Freud (1856-1939).
- Susceptible to analysis in terms of unconscious thoughts or desires.