Some sundowners to loosen up? (4)

I believe the answer is:

undo

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'loosen up?' is the definition.
'undo' can be an answer for 'loosen' (I've seen this before). I am not sure about the 'up?' bit.

'some sundowners to' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'UNDO' is hidden within 'sundowners to'.

Can you help me to learn more?

(Other definitions for undo that I've seen before include "Cancel - destroy" , "Relax" , "Open buttons or ruin" , "Ruin or loosen ties" , "Loosen; cancel" .)

I've seen this clue in The Irish Times.

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