Crossword Genius

Sun's beginning to disappear? Better take top off! (5)

Ross

I believe the answer is:

uncap

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

'take top off' is the definition.
(I've seen this before)

'sun's beginning to disappear? better' is the wordplay.
I cannot quite see how this works, but
'better' could be 'cap' (both can mean to beat somebody else's attempt) and 'cap' is present in the answer.
The remaining letters 'un' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for uncap that I've seen before include "Remove a top" , "Take the top of the bottle" , "Take the lid off" , "Remove a cover from" , "Remove the lid" .)

I've seen this clue in The Independent.
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