Crazy losing head about one occasion (6)

I believe the answer is:

entail

I'm a little stuck... Click here to teach me more about this clue!

'one occasion' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for entail that I've seen before include "Estate settled" , "Involve or require" , "settle estates for the long term" , "Need" , "Cut" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.